To prepare a plum streusel coffee cake recipe, first you need to gather your ingredients. You'll need: 1 cup granulated sugar, 3/4 cup butter, 1 teaspoon vanilla, 1/4 teaspoon cinnamon, 1/2 teaspoon nutmeg, 2 tablespoons buttermilk, and 1 tablespoon vanilla extract. Preheat the oven to 350 degrees F (180 degrees C). Grease and flour two 8-inch round cake pans and set aside. Sift together the flour, cocoa, baking powder, salt and baking soda. Add the dry ingredients to the wet ingredients and mix until just combined. Pour the batter into the prepared pan and bake the cake for 35 to 40 minutes, or until the toothpick inserted in the center comes out clean. Allow to cool before removing from the pan.