Preheat the oven to 350°F. Prepare three 8-inch round cake pans with non-stick baking spray and parchment paper cut into circles on the bottom.
Sift the flour, sugar, cornstarch, baking powder, baking soda and salt together in a large mixing bowl. Whisk well to evenly combine.
Combine the plant-based milk, oil, vinegar and vanilla extract in a medium sized bowl or large measuring glass. Whisk well to emulsify the oil and milk together.
Once the oven has completely preheated, add the wet ingredients to the dry ingredients. Mix until combined. Divide the batter evenly between the prepared cake pans and bake for 35-38 minutes. Don't open the oven early or your cakes may collapse in the middle. The cakes are done once they pull from the sides, and a toothpick inserted into the middle comes out clean or with a few cooked crumbs.
Cool the cakes for 10 minutes on a rack. Gently run a sharp knife along the sides of the cakes to loosen any bits that are stuck. Gently and evenly turn the pans over on a wire cooling rack. Remove the pans. Leave the parchment on until you're ready to assemble the cake. Once completely cooled, gently wrap in plastic and store flat in the fridge.
For the Berry Filling:
Add the frozen berries, sugar and lemon juice to a medium sized pot over medium heat. Stir the mixture until it starts to boil. Reduce the heat to low and allow the filling to simmer for 5-8 minutes. You want the berries to burst to release their juices and concentrate a bit, but you don't want it all to evaporate.
Turn off the heat. Pour the filling into a metal sieve or strainer resting over a medium sized heat-safe bowl. Use a rubber spatula or large metal spoon to press the fruit against the mesh to get as much juice as possible out of the fruit. You'll have about 1/3-1/2 cup of seeds and berry skin – this can be composted or saved for another use like smoothies. Rinse out the pot and pour the strained berry filling back in.
In a small bowl, whisk the cornstarch and water together to make a slurry. Stir until the cornstarch is completely dissolved – any starch that is not dissolved will make lumps in your filling.
Whisk the cornstarch slurry into the fruit filling. Turn the heat on to medium. Whisk constanty to prevent burning and lumps from forming. Once the filling is simmering, reduce the heat to low until the mixture begins to thicken. Cook one more minute once it has thickened. Remove from heat and pour the berry filling into a bowl. Place a sheet of plastic wrap directly onto the hot fruit filling to prevent a skin from forming. Refrigerate for at least 2 hours or until completely chilled.
For the Vanilla Frosting:
In a stand-mixer with a balloon whisk (or with a large bowl and beaters), beat the vegan butter and shortening together until smooth. Add in the sifted confectioner's sugar, one cup at a time. Beat for 3-4 minutes, scraping down the sides as needed.
Add in the vanilla extract and beat another minute. Add the plant-based milk 1 teaspoon at a time beating well each time. The frosting should be smooth and pipeable but not so soft that it slumps. If you add too much milk, you can add more sifted confectioner's sugar a few tablespoons at a time but this naturally will make the frosting sweeter.
To Assemble the Berry Vanilla Cake:
Start with completely chilled cakes so they are firm. Use a large serrated knife to level the tops of each of your cake layers, removing about 1/8-inch or so. Ideally, each layer will be the same height once leveled for a nice even looking cake.
Spread a bit of frosting onto the middle of the cake board (or serving plate) and then center your first layer on it. Spread about 3/4 cup of frosting evenly over the cake, then pipe a frosting dam along the edge to held in the berry curd using an approximately 1/2" round tip. Gently spread about 1/2 cup of the berry curd over the top of the frosting, taking care not to press in and lift any of the buttercream into the berry layer.
Add the next cake layer and repeat until you have your 3 cake layers assembled with the berry layer and frosting. Smooth a thin layer of frosting on the top and around the sides of the cake. Top with an assortment of fresh berries and mint leaves, if desired. The cake is best served at slightly cooler than room temperature but stores best in the fridge considering it's topped with berries. Simply remove it from the fridge about 30 minutes before serving.
Properly Measure Your Flour: For consistency and reliability, it's best to weigh your dry ingredients using a scale. If you don't have one, don't simply scoop the four out of the bag - you'll be taking up to 35% too much which leads to a dense dry cake. Instead, whisk the flour first, then spoon the flour into your measuring cup and then scrape the top of the measuring cup with a knife.