Preheat the oven to 350°F (175°C) and prepare three round cake pans by greasing with butter and lining with parchment paper.

In a large mixing bowl, cream the unsalted butter and granulated sugar together until fluffy, about 3-4 minutes.

Add eggs one at a time, beating well after each addition. Then blend in the vanilla extract.

In another bowl, whisk together the flour, baking powder, and salt. Gradually mix this dry mixture into the wet ingredients until just combined.

Fold in the buttermilk, lemon zest, and lemon juice gently.

Coat the blueberries with a bit of flour or cornstarch and gently fold them into the batter.

Divide the batter among the prepared pans and bake for 21-25 minutes until a toothpick comes out clean.

Let the cakes cool for about 10 minutes in the pans, then turn out onto wire racks to cool completely.

While the cakes cool, beat the cream cheese and butter together until smooth. Gradually add confectioners sugar, milk, lemon extract, and sea salt.

Once the cake layers are cool, frost between the layers and the top and sides of the cake.

Garnish with fresh blueberries and lemon slices before serving.

Chill the frosted cake for at least 2 hours before serving for the best flavor.