For cake, grease a 13x9x2-inch baking pan; set aside. In a large bowl, stir together flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt. Add eggs, the 3/4 cup milk, the oil, and the 1 teaspoon vanilla. Beat with an electric mixer on low speed until combined, then on medium speed for 2 minutes.
Dissolve coffee crystals in the cold water; add to cocoa mixture. Beat on low speed until smooth (batter will be thin). Pour into prepared pan. Bake in a 350 degree F oven for 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack.
For frosting, in a medium bowl, beat together cream cheese and peanut butter (mixture will become thick). Gradually beat in the 2 tablespoons milk and the 1/2 teaspoon vanilla. Beat in enough of the powdered sugar to make of spreading consistency. Spread on cake. Sprinkle with chocolate pieces, if desired. Makes 15 servings.