Fluffy lemon buttermilk cake layers with blueberry jam filling and creamy lemon buttercream.
Preheat the oven to 325 °F. Spray three 6-inch round cake pans with non-stick spray, line the bottom of each pan with parchment paper.
In a medium-size bowl, whisk and stir together the cake flour, baking powder, and salt. Set aside.
In the stand mixer bowl fitted with a paddle attachment, beat the butter, sugar, and oil on medium-high for 5 minutes.
Add the eggs, one at a time to the stand mixer bowl and beat on medium until well incorporated.
Turn the mixer on low, add the flour mixture, followed by the buttermilk.
Pour in lemon juice, vanilla, and zest. Mix on medium for another 30 seconds then scrape down the sides of the bowl.
Evenly distribute the cake batter among the three cake pans and bake for about 35-45 minutes or until a toothpick inserted into the center of each cake comes out clean or with just a few moist crumbs.
Remove cakes from the oven and let cool in pans for 10 minutes before inverting onto cooling racks to cool completely — wrap cakes in plastic wrap and place in the freezer to chill for at least 30 minutes.
In a medium saucepan, mix the blueberries and sugar. Continue mixing with a rubber spatula over low-medium heat until the sugar has dissolved and the berries begin to boil.
In a small bowl, combine the cornstarch and water. Add cornstarch mixture to the boiling berries and continue to stir with a rubber spatula.
Continue cooking the mixture for 2-3 minutes until it is thickened and coats the back of a spatula. Remove from heat and add lemon juice.
Pour the blueberry filling into a small bowl to cool. Press plastic wrap directly onto the surface to prevent a skin from forming. Set aside to fully cool.
In a small saucepot, whisk together lemon juice and cornstarch. Place the pot over medium heat for 2-5 minutes to bring to a boil, stir with a rubber spatula constantly to prevent it from burning on the bottom.
Add lemon zest to the mixture. The lemon should be opaque and thick. Once thick, remove from the heat and place into the freezer to cool quickly.
In the stand mixer bowl fitted with the paddle attachment, beat the butter until light and fluffy, about 2 minutes.
Turn the mixer to low and add the cold lemon gel, salt, and vanilla.
Next, gradually add powdered sugar, followed by the heavy cream. Mix on medium-high speed for about 5 minutes, until light and fluffy. Scrape down the sides of the bowl.
Place ⅔ cup buttercream into a piping bag fitted with #844 star tip.
Before using, turn the mixer on low and mix for 2 minutes to push out any air bubbles or use a rubber spatula to push out the air bubbles. (You may need to repeat this step when frosting the outside of the cake).
Remove the lemon cake layers from the freezer and unwrap the cake layers. Next, use a serrated knife to cut off the dome of the cakes to make the layers flat and even.
Add a small amount of buttercream onto the center of the cake board.
Layer a chilled lemon cake, cut side up onto the cake board. Place a thin layer of buttercream on top of the cake then pipe two rings of buttercream stacked on top of one another. Add 1/4 cup of blueberry jam inside the ring of buttercream.
Place another cake layer cut side down over the top. Repeat the process for another layer—Crumb coat and freeze for 30 minutes.
Next, fill a piping bag that's fitted with a #844 star tip with lemon buttercream.
On the frozen cake, start from the bottom while spinning the cake, squeeze consistent pressure on the piping bag, and pipe a row around the cake. Then pipe another row directly on top of the previous row. Make sure to line up the starting point for a straight seam in the back of the cake.
Repeat this process all over the sides of the cake.
Then with the same piping bag and tip pipe, a rope border on top of the cake, leaving the center open.
Finally, fill in the top of the cake with blueberry jam. Serve at room temperature and enjoy!