How to Make It
Preheat the oven to 350° and line 2 large baking sheets with parchment paper. In a medium bowl, whisk the flour with the cocoa powder, baking soda, salt and cream of tartar. In a large bowl, using a handheld electric mixer, beat the butter with the sugar at medium speed until fluffy, about 2 minutes. Beat in the eggs and vanilla. At low speed, beat in the dry ingredients and milk in 3 alternating additions; scrape down the side and bottom of the bowl as necessary.
Using a 2-tablespoon ice cream scoop, scoop 10 level mounds of batter onto each baking sheet, about 2 inches apart. Bake the cakes in the lower and upper thirds of the oven for about 12 minutes, until risen; shift the pans from top to bottom and front to back halfway through baking. Transfer the cakes to racks and let cool completely. Repeat with the remaining batter, allowing the baking sheets to cool between batches.
In a large bowl, using an electric mixer, beat all of the ingredients except the crushed candy canes until thick and smooth, 1 to 2 minutes. Scoop 3 tablespoons of the filling onto the flat side of 20 cakes and close with the remaining cakes, pressing the filling to the edges. Roll the edges in crushed candy canes, if desired, and serve.