Melt the chocolate with your favorite method (double boiler, microwave, or large bowl set over simmering water). Stir until smooth and set aside, stirring occasionally.
In a bowl, combine flour, baking soda, salt, and nutmeg. Set aside.
In the bowl of your electric mixer, cream butter and sugar until light and fluffy, stopping the mixer and scraping down the bowl to best combine the ingredients.
Add eggs one at a time, beating well after each addition.
Set the mixer speed to low and add the flour mixture and the buttermilk alternately, beginning and ending with the flour.
Stir in vanilla extract and cooled chocolate, and blend until the color is even.
Pour into pan(s), level, and then move some batter from the center to the edges for the best, most-even rise.
Bake in the lower 1/3 of the oven for 40-50 minutes for the sheet cake or 35-45 minutes for cake pans. Always set your timer for the low end of the range and check it for doneness. Bake until a cake tester inserted in the center comes out clean. When you gently touch the middle, it will spring back slightly.
Remove to cooling rack(s) for 10 minutes. Invert onto plate, if desired.