Add the flour and sugar to a small bowl. Whisking constantly, pour in the eggnog. Pour the mixture through a fine mesh sieve into a small pot, pressing any lumps through the sieve with the back of a spoon. Whisking constantly, heat over medium-low. Once the mixture thickens and is bubbling, cook another 2 minutes, continuing to whisk. Ensure the mixture continues to bubble for the whole time or the frosting will taste like flour and will not thicken properly. Whisk in the vanilla, rum and nutmeg.
Pour the flour mixture into a bowl and place plastic wrap directly over the surface. This will prevent a skin from forming. Let cool completely to room temperature – it will have a thin pudding or caramel consistency.
In a large bowl with a hand mixer, (or in a stand-mixer with whisk attachment), beat the vegan butter on high for 2 minutes, scraping down the sides as necessary. Reduce the speed to medium, and gradually spoon in the cooled flour mixture. Once it’s all added, beat on high for about 5 minutes or until light, fluffy and silky. Taste to see if you’d like to add the additional nutmeg and rum extract (or rum). If adding, beat the frosting for 3 more minutes after the addition. See Notes below for trouble shooting.
Vegan Molasses Cookies
Preheat the oven to 350°F/177°C/gas 4. Line a cookie sheet with a silicone baking mat or parchment paper.
Sift the flour, cornstarch, baking soda, cinnamon, salt, ginger, cloves and nutmeg into a medium sized bowl. Whisk to combine.
In a large bowl with a hand-mixer (or stand-mixer bowl with paddle attachment), beat the brown sugar and vegan butter together on medium speed until combined and fluffy. Add in the molasses, milk and vanilla, beating until well mixed. Add in the flour mixture and beat at a low speed until evenly combined. Cover and refrigerate for about 30 minutes – this helps the dough be less sticky. However, the dough can be baked straight away if you're short on time and don't mind working with sticky dough.
Place the sugar in a small bowl. Scoop the dough into evenly sized 1.5 tablespoon pieces (it's easiest to use a small cookie scoop). Making them the same size results in symmetrical sandwich cookies later. For those using a kitchen scale, each cookie will be 18-19 grams. Roll the cookie dough into 1-inch (2.5 cm) balls and then roll each ball in the sugar. If your dough is sticky, sprinkle the sugar over the cookie ball then roll it. See the Notes below on making extra large sandwich cookies.
Place the cookie balls on the prepared cookie sheet about 2 inches (5 cm) apart. Slightly flatten the tops of each cookie dough ball with the back of a clean, dry measuring cup. Bake for 7-9 minutes or until the edges are just set – DO NOT OVERBAKE. The cookies will continue to cook once removed from the oven. Remove from the oven. If the cookies don't have crackles, let the cookie tray drop from about an inch (2.5 cm) above a heat safe surface. Repeat 2-3 times then let the cookies cool for 10 minutes on the tray. Remove from the tray and continue to cool on a wire rack. Cool completely before frosting.
To Assemble the Molasses Sandwich Cookies:
Fill a piping bag fitted with either an open star or round tip with the eggnog frosting and pipe the frosting onto the bottom of one cookie. Pipe in a single layer, creating a swirl - you can start from the middle or the outside edge, whichever you find easiest. Place a second cookie evenly over the frosting, crackle side up. Don't squeeze down or the frosting will squish out. Repeat with the rest of the cookies and frosting. Place the cookie sandwiches on a tray in a single layer into the refrigerator. If you stack the cookie sandwiches on top of each other before the frosting has set up, it may squish out of the sides of the cookies. The frosting will firm up once chilled.Let them sit at room temperature for about 20 minutes before serving if you like the frosting to be softer. Otherwise, these vegan cookie sammies are amazing chilled!
Storage Recommendations:Once assembled with the vegan eggnog icing, these vegan cookie sandwich keep well in the refrigerator in a sealed container for about 5-7 days (the refrigerator is safest because of the higher moisture content in the frosting compared with American buttercream.) They can be frozen in an airtight container for about 3 months. Let them thaw in the refrigerator overnight.Make These Sandwich Cookies Gluten-Free:Switch out the wheat-based flour with an equal amount of 1:1 gluten-free baking mix which contains xanthan gum. See blog post for recommendations.For a gluten-free ermine buttercream, you can also substitute all-purpose flour with a 1:1 gluten-free flour blend. The results may not be quite as silky but will be just as delicious!Eggnog Frosting Troubleshooting:If curdling happens, your frosting is too cold. Place the bowl over steaming hot water until the vegan butter just starts to melt. Try whipping again. If it still isn't pulling together, add another tablespoon of room temperature vegan butter and whip again.If your frosting is too soft and slouching, it is too warm either from handling or the kitchen being hot. Place the bowl in the fridge for 10-15 minutes at a time until it firms up.Don't Have a Piping Bag or Tips?If you don't have a piping bag or piping tips, you can use a large ziplock type plastic bag. Fill the bag with frosting and snip off a corner so the opening is roughly ¼-inch (½ cm). Otherwise, you can spread the frosting on with a knife in a pinch!Extra Large Sandwich CookiesThis recipe yields about 18 - 2" (5 cm) sandwich cookies if using 1.5 tablespoon (18-19 grams) of cookie batter (or size small cookie scoop), or 9 - 4" (10 cm) cookie sandwiches if using 3 tablespoons (36-38 grams) of cookie dough (or size medium cookie scoop.) Give each large flattened cookie dough ball 3" (7.5 cm) of space. Baking time will be a bit longer, 9-11 minutes.