Line cake pan with paper. Spray 8 inch cake pan that is 4 inches tall and deposit 4# batter into pan. Bake until cake springs back lightly when touched. Let cake cool completely and de-pan.
Trim the belly of the cake and cut into three equal sized layers. Mix a bit of coconut extract into the buttercream and place in a piping bag. Pipe buttercream around the edges of the cake round. Fill with key lime filling. Repeat with the next layer, place final cake layer on top. Use the piping bag to place some buttercream around the sides and top of the cake. Crumb coat the cake. Mix a bit of green food coloring and lime extract into flat icing. Slightly warm the icing then pipe it in a circular motion to cover the top of the cake, ending with going just over the edges to create a drip effect. Let icing set completely and decorate with fresh coconut, sliced key limes, and coconut truffles.