Make the Macaroons: Preheat the oven to 300 degrees F.
Line several sheet pans with parchment paper and set aside. In a bowl, combine 2 1/2 cups of the sugar, almonds, and cocoa powder.
In another clean dry bowl whip the egg whites with the remaining 1/2 cup sugar until medium peaks form (do not beat to stiff peaks). Fold egg whites into dry ingredients to a make a meringue. Fill the pastry bag with the meringue. (To easily fill the pastry bag, place it (with a clothespin holding it shut at the bottom) in a tall round container such as a coffee can, mayonnaise jar, or vase, folding the edge over the rim of the container.)
Pipe rows of kisses 1-inch in diameter (about the size of a quarter), keeping them 1/2 inch apart, onto the sheet pans. Bake until very light brown, rotating the pans after 8 minutes, about 17 minutes. Let the macaroons cool on the pan.
Meanwhile, make the Peanut Butter Buttercream: In a standing mixer fitted with a whisk attachment, cream the butter until light and fluffy. Add the peanut butter and whip. Add the confectioners' sugar and vanilla and whip until fluffy. Transfer the buttercream to a clean pastry bag fitted with a round tip.
Peel 2 macaroons off the parchment, pipe the buttercream on 1 flat side and sandwich the other flat-side down on top. Repeat with the remaining macaroons. Keep chilled in an airtight container until ready to serve.