Before icing the cake, be sure it is completely chilled. Chef Elizabeth Chambers often stick it in the freezer for a bit before icing it.
- Preheat oven to 275 degrees and grease an 18x13-inch sheet pan.
- In large bowl, combine flour, 1¼ cups sugar, salt, baking soda, and baking powder. Set aside.
- In bowl of stand mixer fitted with whisk attachment, whisk together buttermilk, yolks, oil, and vanilla. Slowly incorporate melted butter.
- Pour dry ingredients into creamed butter mixture and mix on low until combined. Set aside.
- In large bowl, whisk egg whites, slowly incorporating remaining ¼ cup granulated sugar until egg whites peak. Fold peaked whites into batter and mix on low until mixture comes together.
- Pour batter evenly into greased cookie sheet. Bake for 45-60 minutes. Cool completely on wire rack. Once cooled, wrap in plastic and refrigerate until ready to frost.
- While cake is cooling, make buttercream by beating butter in large bowl until smooth. Slowly incorporate confectioners’ sugar. Add cream of coconut and coconut flakes to mixture and beat for 3 minutes or until fluffy. Evenly spread buttercream over cooled cake.