Cut 1/4 inch off lemon ends; reserve 5. Cut each lemon into 12 paper-thin slices.
Bring sugar and 2 cups water to a rolling boil in a medium-size, heavy skillet, swirling skillet to dissolve sugar. When liquid is clear and bubbly, reduce heat to medium-low. Add lemon slices and reserved ends in a single layer. Simmer until rinds are translucent, 30 to 35 minutes. Transfer to baking sheets lined with parchment paper. Let cool completely.
Fill a pastry bag fitted with a 1/4-inch round tip (such as Ateco #10) with buttercream. Pipe buttercream into center of 5 slices of varying size. Stack, starting with smaller slices, increasing to larger ones, and then decreasing to smaller ones again. Top with an end piece. Make a hole in top of end piece with a skewer; insert stem of a lemon leaf. Repeat to make 4 more toppers. Candied lemons can be refrigerated up to 2 days.