Fill cake. Remove cooled cake from pan. Cut cake in half vertically for two 6 1/2 inch x 4 1/2 inch sections. Cut in half horizontally to create two equal layers.
Remove top layer of cake from both sections; set aside. Spread lemon curd on bottom cake sections; replace with top halves of cake, cut side down.
Lightly ice top of cake with buttercream icing. Chill until buttercream has crusted and cake is well-chilled, about 10 to 15 minutes.
Cut cake into 1 in. squares.