Preheat the oven to 350°F and line two 8-inch nonstick cake pans with parchment paper. (If you don’t have nonstick cake pans, grease the pans with cooking spray or coconut oil, then use about 1 tablespoon of extra flour per pan to lightly flour them.)
In a large bowl, whisk together the flour, sugar, baking powder, and baking soda.
In a medium-sized bowl, mix together the dairy-free milk, coconut oil, applesauce, lemon zest, lemon juice, and vanilla extract.
Fold the wet mixture into the dry until combined, making sure there are no clumps of the dry mixture left.
Fold in the blueberries.
Evenly divide the batter between the prepared cake pans and place them in the oven to bake for 33 to 35 minutes or until a toothpick comes out clean when inserted in the middle. The cakes should be completely set and bounce back at a light touch.
Remove the cakes from the oven and carefully invert them onto a cooling rack, then turn them right side up. Let the cakes cool completely; this should take about 30 minutes.
Use a hand mixer to mix the buttercream with the blueberry jam until well combined. Cream the vegan butter until fluffy, about 3 minutes. Then add in 1 cup of powdered sugar at a time, mixing in between. Spoon in the blueberry jam, along with the vanilla, and continue mixing until combined. If the frosting is looking like it could start to split, add in another cup of powdered sugar.
When ready to frost, place one cake on a cake plate or stand. Evenly spread about ⅔ cup of the blueberry frosting on the top of the first layer. Place the second cake on top and frost the top with about ½ cup of the blueberry frosting. Use the remaining frosting to generously frost the edges and sides of the cake (you may have some frosting leftover). Garnish with fresh blueberries and lemon wedges.
Slice and serve. Store the cake in an airtight container in the refrigerator for up to 5 days or the freezer for up to a month; store any leftover frosting in an airtight container in the refrigerator for up to a week.
To prepare this cake a day ahead, complete Steps 1 through 7. Then place the cooled cakes on separate plates, cover them with plastic wrap, and store at room temperature until you’re ready to frost the next day.