Place water and sugar in a small saucepan to make a simple syrup. Bring to a boil and continue to simmer until sugar has dissolved. Let cool before using. Place one cake layer on cake turntable. Use a serrated knife to go around the top edge of the cake layer to score it before leveling off the domed cake top, so you get a flat, even cake layer. Once scored and the height is the same around the cake, use the serrated knife in a sawing motion, going back and forth slowly to remove the cake dome (cake scraps make an excellent trifle!). Repeat for all four layers. Place first layer of cake on turntable. Use a pastry brush to moisten the top of the layer with syrup evenly. Spread ¾ cup buttercream on top of cake with a small offset spatula. Make sure to spread all the way to the edges of cake (but not on the sides). Place next cake layer on top, gently pressing down to make sure cake is even and secure. Repeat with each layer. Spread 1 cup buttercream on outside of cake, working the small offset spatula back and forth to cover entire exterior of the layered cake. Crumbs may fall off, but that’s okay, the buttercream will act as a glue to adhere crumbs to the cake. Clean up and smooth the surface by keeping the offset spatula flat on top while spinning the cake turntable. For the sides, hold the offset spatula upright and turn cake turntable to smooth buttercream. This is the crumb coat. Chill in refrigerator 30 minutes. Remove chilled layer cake. Spread ¾ cup buttercream on top and sides of cake. Use small offset spatula to apply final coat of frosting. Decorate as desired.