Berry Buttercream

This berry buttercream compliments your dessert marvelously.

Purée the berries in a food processor or blender until they are as smooth as possible. Press the purée through a fine-mesh strainer, discarding the skins and seeds. You’ll have about ⅓ to a scant ½ cup of berry purée. Set aside.
In a large bowl, beat the butter until it is light and fluffy. Beat in the sugar, 1 cup at a time, beating well after each addition. Add ¼ cup purée and beat until the color is fully incorporated. If you’re going for a bluer berry tint than the blueberries will provide, add a single drop of liquid blue food coloring to the blueberry-purée buttercream. Keep the frosting covered until it is needed.
To create the frosting design in the photos: I made four quarter-batches of this frosting, each one with 4 tablespoons (½ stick) butter, 1 cup confectioners’ sugar, and 1 tablespoon berry purée. The first batch was blackberry, the second was raspberry, the third was blueberry, and the fourth was also blueberry, but blueberry with a drop of blue food dye. The frosting was piped with a small round pastry tip (about ¼-inch opening) in horizontal lines across the cake, alternating the frostings in stripes*. When the top of the cake is covered, drag a knife crosswise through the frosting (but not into the cake, or you’ll pick up crumbs) in 1-inch intervals; the first drag should go in one direction, the second will go in the other. When you’re done, your cake top will look like a Mossimo Zig Zag dress—you know, an affordable one.