Devil’s Food Layer Cake

1 cup boiling water 

3/4 cup unsweetened natural cocoa powder 

1 3/4 cups all-purpose flour 

1 1/2 tsp. baking soda 

1/4 tsp. fine sea salt 

2 cups granulated sugar 

10 Tbs. (1 1/4 sticks) unsalted butter, at room temperature 

3 eggs 

1 tsp. vanilla extract  

1 1/4 cups buttermilk 

3 3/4 cups confectioners’ sugar 

1 cup unsweetened natural cocoa powder  

8 Tbs. (1 stick) unsalted butter, at room temperature 

1 tsp. vanilla extract  

1 cup heavy cream, or as needed