Preheat the oven to 350°F. Butter and flour three 6-inch round cake pans.

Beat together the wet ingredients in a medium bowl. Set aside.

Sift the dry ingredients together in a medium bowl. Remove one tablespoon and add it to a small bowl with the blueberries. Toss and set aside.

In a large mixing bowl, beat the softened butter on medium speed until creamy, about 1 minute. Add the flour mixture and beat on low speed until sandy.

With the mixer running on low speed, slowly pour in the wet ingredients and mix until just combined, then beat until fluffy.

Fold in flour-coated blueberries by hand with a spatula.

Divide the mixture evenly into the cake pans.

Bake for about 30 minutes or until springy. Cool in pans for 15 minutes, then on a wire cooling rack.

Place ½ cup of blueberries in the freezer.

In a small saucepan, combine the remaining blueberries, sugar, lemon juice, and water. Cook until the berries burst. Mash and stir, then simmer until reduced by half.

Pour the reduction through a fine-mesh strainer and chill.

Beat the butter and salt on medium speed until fluffy, about 3 minutes. Gradually add in confectioners' sugar.

Beat in lemon juice until desired consistency and taste are reached.

Transfer about 1 cup of the frosting to mix in blueberry reduction until desired flavor.

Transfer the blueberry buttercream to a piping bag.

Place the rest of the lemon buttercream in a piping bag and snip off the tip.

Pipe a thick ring of lemon buttercream around the edge of one cake layer. Pipe blueberry-lemon buttercream in the center. Top with another layer and repeat.

Pipe more lemon buttercream around the sides and top of the cake and smooth it.

Pipe lemon buttercream dollops about 1 inch from the edge. Add frozen blueberries inside. Refrigerate for 20 minutes before slicing.