Microwave the buttermilk for 30-45 seconds, just until warmed. Stir in the instant coffee granules until dissolved, then set aside.

In a large mixing bowl, cream the softened butter and brown sugar together until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract. In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, salt, and ground nutmeg. Gradually add the dry ingredients to the creamed mixture, alternating with the buttermilk mixture, mixing until just combined.

Pour the batter into a greased 13x9-inch baking pan, spreading it evenly. Bake at 350°F for 20-25 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan on a wire rack for about five minutes.

Using the handle of a wooden spoon, poke evenly spaced holes throughout the cake, about two inches apart. Slowly pour ½ cup of the caramel topping into the holes, allowing it to soak in. Drizzle the remaining caramel over the surface of the cake. Let the cake cool completely before frosting.

To make the frosting, stir the baking cocoa and instant coffee granules into the boiling water until dissolved. Let the mixture cool to room temperature. In another bowl, beat the softened butter with the confectioners’ sugar until light and fluffy. Stir in the melted chocolate and the cooled cocoa mixture until smooth and well combined.

Spread the frosting evenly over the cooled cake. Slice and serve.