Microwave buttermilk for 30-45 seconds or just until warmed.
Stir in coffee granules until dissolved.
In a large bowl, cream butter and brown sugar until light and fluffy.
Add eggs, one at a time, beating well after each addition.
Beat in vanilla.
Combine the flour, cornstarch, baking powder, baking soda, salt and nutmeg; add to creamed mixture alternately with buttermilk mixture, beating well after each addition.
Transfer to a greased 13x9-in. baking pan.
Bake at 350 degrees for 20-25 minutes or until 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.
Pour 1/2 cup caramel topping into holes.
Spoon remaining caramel topping over cake, cool completely.
For frosting, in a small bowl, stir cocoa and coffee granules into boiling water until dissolved; cool to room temperature.
In a small bowl, cream butter and confectioners' sugar until light and fluffy.
Stir in melted chocolate and cocoa mixture until well combined.
Serve and enjoy!