Preheat oven to 350°. Line a 18×13-inch rimmed baking sheet with parchment paper. Spray sides of pan with baking spray with flour.
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In a medium bowl, whisk together buttermilk, hot coffee, oil, egg, and vanilla extract. Fold buttermilk mixture into flour mixture until fully combined. Pour batter onto prepared baking sheet.
Bake until a wooden pick inserted in center comes out clean, 15 to 20 minutes. Let cool in pan for 10 minutes. Remove from pan, and let cool completely on a wire rack.
Using a 2-inch round cutter, cut 24 circles from cooled cake.
Place Italian Buttercream in a piping bag fitted with a large open star tip (Wilton #4B). Pipe Italian Buttercream onto 12 cake circles. Top each with a remaining cake circle. Pipe remaining Italian Buttercream on top of cake circles. Top each cake with a blackberry.
Notes
MAKE-AHEAD TIP: Cakes can be prepared and assembled several hours in advance, stored in an airtight container, and refrigerated. Let come to room temperature before serving.
Recipe by TeaTime Magazine at https://teatimemagazine.com/mini-chocolate-cakes/