Preheat the oven to 350F/177C. Line two, 8-inch (20-cm) cake pans with parchment paper and spray the sides with a baking spray.
In a large mixing bowl, combine the buttermilk, oil, eggs, sugar and extracts. Beat on medium speed for 2 to 3 minutes, until the mixture is smooth. In a separate bowl, combine the flour, baking soda, baking powder and salt. Mix the dry ingredients together, then sift into the cake batter and mix on medium speed for about 1 minute, just until incorporated.
Next, zest 2 large lemons and add the zest into the batter. Cut the lemons open and squeeze out the juice into a measuring cup. Strain the juice through a fine mesh strainer and add 2 tablespoons (30 ml) to the cake batter.
Pour the remaining 2 tablespoons (30 ml) of lemon juice over the blueberries, arranged in a medium-sized bowl. Add the cornstarch and use a spoon to toss the berries until they’re well-coated.
Add just half of the blueberries into the cake batter and use a spatula to fold them in, along with the zest and lemon juice.
Divide the cake batter evenly between the two prepared pans. Sprinkle the remaining blueberries on top. Bake the cake layers for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean. Remove the cake layers from the oven, out of the pans and onto a wire rack to cool completely.
Making the Lemon Frosting:
Meanwhile, prepare the lemon cream cheese frosting. Place the softened butter into a mixer bowl and attach a flat beater. Beat the butter on medium to high speed for 6 to 8 minutes, until it’s white and creamy. Add in the cream cheese and continue mixing on medium speed for 4 to 6 minutes, scarping down the sides of the mixing bowl occasionally, until the mixture is smooth.
Add in the confectioner’s sugar, vanilla extract and lemon extract. Continue mixing on medium-high speed for a couple more minutes, until the frosting is smooth. Add the lemon zest last and mix for 30 seconds. Place the frosting into the refrigerator to chill.
Assembling the Cake:
Once the cake layers have cooled completely, use a long, serrated knife to level off the top (if needed) and split the layers in half.
Transfer about 2 cups of the cream cheese frosting into a pastry bag tipped with a simple round tip, or a star tip. To assemble the cake, spread about ¾ cup of cream cheese frosting onto each cake layer. Next, pipe a barrier ring of frosting around the edge of the cake; watch my video to see how it’s done. Into the center, add 3 tablespoons of lemon curd or blueberry preserves.
Repeat the same process for the next 2 layers, then add the final layer on top. Frost the top and sides of the cake with the remaining frosting, smoothing it with a spatula and cake scraper. Refrigerate the cake for about 30 minutes.
To garnish the top, pipe dollops of cream cheese frosting along the edge, then top with fresh blueberries and sliced lemon. Allow the cake to chill in the refrigerator for 2 to 4 hours before serving.