Preheat oven to 275 degrees F (135 degrees C). Grease a large, rimmed baking sheet with butter.
Mix flour and baking powder in a bowl.
Beat 1 3/4 cups sugar and eggs in a large bowl with an electric mixer until well combined. Add flour mixture and buttermilk alternately in 3 or 4 batches, beating after each addition on low speed until well combined. Spread batter onto the prepared baking sheet, smoothing out with a spatula.
Mix coconut flakes with 1/2 cup sugar and sprinkle evenly over the batter.
Bake in the preheated oven until set and lightly browned, about 30 minutes. Remove from oven and immediately pour heavy cream all over the cake while cake is still hot. Cool completely for about 1 hour, then cut into squares or rectangles.