Microwave the buttermilk for 30-45 seconds or just until warmed. Stir in coffee granules until they are dissolved.
In a large bowl, cream the butter and brown sugar until the mixture is light and fluffy. Add eggs, one at a time, beating well after each addition. Then, beat in vanilla.
In another bowl, combine the flour, cornstarch, baking powder, baking soda, salt, and nutmeg. Once it is fully mixed together, add it to the creamed mixture alternately with buttermilk mixture, beating well after each addition.
Pour the coffee cake mix into a greased 9×13 inch baking pan. Bake at 350 degrees for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack for 5 minutes.
Using the end of a wooden spoon handle, poke holes in cake. You can use a fork for smaller holes, but we like the larger holes.
Pour ½ cup caramel topping into holes. I always spoon the last bit of caramel topping over the cake. Once you have added the topping, let it cool completely so that the cake soaks up the goodness.