Preheat the oven to 350F. Spray a baking sheet with nonstick spray.
Whisk together the flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
In a large bowl, combine the granulated sugar and lemon zest and work the zest into the sugar with your fingers.
Add the eggs to the sugar mixture and whisk until foamy and lighter in color, 1 to 2 minutes.
Whisk in the vegetable oil, vanilla extract, lemon extract, and lemon juice.
Gradually stir in the flour mixture, alternating with the buttermilk, whisking well after each addition.
Transfer the batter to the prepared pan. Bake until the cake is lightly golden and a toothpick inserted into the center comes out clean, 16 to 18 minutes, rotating the pan halfway through.
Let cool completely in the pan on a rack, at least 1 hour.
Using a circle cutter, cut out circles for your mini cake layers.
Frosting:
Beat the butter in a stand mixer fitted with the paddle attachment on medium speed until smooth, about 2 minutes.
Add the preserves (or freeze dried blueberry powder) and lemon juice and beat on low speed until just combined, 30 seconds.
Increase the speed to medium and beat until the butter and preserves are well combined, 1 to 2 minutes.
Gradually add the powdered sugar and beat on low speed until combined.
Scrape down the bottom and sides of the bowl, increase the speed to medium and beat until smooth and fluffy, 3 to 4 minutes.
Assembling the mini cakes:
Add the finished blueberry frosting into a piping bag, with or without a decorating tip.
Grab one layer of the mini cake and add a layer of frosting to the top.
Place the second cake layer on top of the first and add more frosting to the top.
Place fresh blueberries on top as a garnish and place on a serving tray.
Notes
These can stay fresh in the refrigerator for up to 3 days.