Preheat the oven to 190°C.
Grease and line a 16-cup mini cake tin or grease 16 5cm diameter chefs' rings on a lined baking tray.
Beat butter, sugar, eggs, flour, baking powder, and coffee essence until smooth.
Fold the chopped walnuts into the batter.
Divide the mixture equally into the tins and bake for 12-15 minutes.
Cool the cakes in the tin for a few minutes before transferring to a wire rack to cool completely.
Beat together the filling butter, icing sugar, milk, and coffee essence until smooth.
Trim the cake tops, slice horizontally, and sandwich with a quarter of the buttercream.
Spread a thin layer of buttercream around the sides of each cake and roll in finely chopped walnuts.
Pipe a buttercream rose on each cake and top with a chocolate coffee bean.