Step 1

Make the cakes: Preheat oven to 350°F/180°C. Sift together flour, cocoa powder, baking soda, and salt. Set aside. Line 2 baking pans with parchment paper.

Step 2

In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy, about 3 minutes. Add egg, and vanilla and mix until combined, then mix in buttermilk. Slowly add dry ingredients, and mix just until combined.

Step 3

Scoop mounds of batter, 1 and 1/2 tablespoons in size (I use this ice cream scoop), onto prepared baking sheets, spacing them 2-3 inches apart. Bake for 10-12 minutes or until the tops spring back when touched. Allow to cool completely on a wire rack.

Step 4

Make the filling: Place all ingredients in a large bowl and beat with a mixer until creamy and fluffy. If the filling is too stiff, you can add a little heavy cream or milk.

Step 5

Assemble the cookies: Spread filling on the flat side of half the cookies. Sandwich with remaining cookies, pressing down slightly so that the filling spreads to the edge of the cookies.

Step 6

Store in the fridge, well covered, for up to a week. Serve at room temperature.