Preheat your oven to 350°F (175°C) and grab three 9-inch round cake pans, lightly greasing them with cooking spray and lining with parchment paper.

In a large mixing bowl, prepare the box vanilla cake mix according to package instructions, adding in the lemon juice and zest.

In a small bowl, toss the fresh blueberries with a tablespoon of all-purpose flour and fold them into the cake batter.

Divide the batter evenly among the prepared cake pans and bake for 18-20 minutes until a toothpick comes out clean.

Cool the cakes in the pans for 10 minutes before inverting onto wire racks to cool completely.

Create the buttercream frosting by beating the softened butter and powdered sugar until fluffy, then adding lemon juice, zest, cream, vanilla, and salt.

Once cool, assemble the cake by layering with frosting in between, then frost the top and sides.

Garnish with extra blueberries and thin lemon slices before serving.