Preheat oven to 325 degrees. Butter six 4-by-2-inch round cake pans, line with parchment, and butter parchment; dust with flour, tapping out excess. Whisk together flour, baking powder, and salt in a medium bowl.

With an electric mixer on medium speed, beat butter and granulated sugar until pale and fluffy, 3 to 5 minutes. Add eggs, 1 at a time, beating well after each addition. Mix in zest and vanilla. Add flour in 3 batches, alternating with 2 batches of buttermilk; mix until just combined.

Divide batter among prepared pans. Bake until golden brown and a cake tester comes out clean, about 30 minutes. Transfer pans to wire racks to cool completely.

With a serrated knife, trim tops of cakes to level; split horizontally to make a total of 12 layers.

Spread 1/2 cup buttercream on each side of 6 bottom layers, and top with remaining layers. Spread a thin layer of buttercream over each cake to form a crumb coat; refrigerate until firm, about 30 minutes. Spread a second coat of buttercream over each cake; smooth with an offset spatula. Refrigerate 30 minutes (or up to 2 days).

Set cakes on a wire rack set over a piece of parchment. Stir together edible glitter dust with confectioners' sugar to achieve desired color; the more sugar used, the lighter the hue.

Cut strips of acetate to preferred width (ours are 1/2-inch wide) and rest them on top of the cakes to create a design. Parallel stripes are the easiest. You can also crisscross stripes, or cover half the cake to create a half-moon. Sift glitter dust over each cake. Carefully lift pieces; reuse to decorate remaining cakes.