For the cookies: Preheat the oven to 300 degrees F. Line a baking sheet with parchment paper. Fit a pastry bag with a round tip (preferably 3/8 to 1/2-inch in diameter). In a large mixing bowl, combine powdered sugar, almond flour, cocoa, and salt. Using an electric mixer, whip the egg whites and food coloring until the whites hold a soft peak. Add the granulated sugar and continue beating whites until they hold a firm peak. Using a large rubber spatula, gently fold egg whites into dry ingredients. After combined, move the rubber spatula back and forth a few times through the mixture until the batter looks slightly shiny. Fill pastry bag with mixture. Pipe 1-inch rounds approximately 1 inch apart. Let cookies sit out at room temperature for 20 minutes. Prepare filling (see instructions below). Bake cookies for about 25 minutes, or until the tops of the cookies feel dry and crisp. Remove cookies from oven and let cool completely on the baking sheet. Once cool, use a spatula to separate cookies from parchment paper.
For the filling: Combine all filling ingredients and, using an electric mixer, beat until smooth and fluffy. Using a small spatula or knife, spread the flat side of half of the cookies with a dollop of filling, about 1/2 teaspoon each (you may have some leftover filling). Sandwich the remaining halves to the filling side.