Preheat the oven to 350 degrees. Grease and flour three, 9-inch round cake layer pans.
Melt the white chocolate in the hot water; allow to cool to room temperature.
Cream the butter and the sugar together until light and fluffy. Add the egg yolks, one at a time, beating well after each addition. Add the melted white chocolate and the vanilla.
Sift the flour and baking soda together and add alternately with the buttermilk to the creamed mixture. Gently fold in the stiffly beaten egg whites. Pour into the prepared pans and bake for 25 to 30 minutes, or until a toothpick inserted in the middle of the cakes comes out clean. Cool for 10 minutes in the pans and then turn out onto wire racks to finish cooling completely. Frost and fill as desired.
Source: “Homemaker’s Cookbook-Cakes”, The Progressive Farmer, 1971