Combine the hot water and instant coffee. Set aside.
Whisk together flour, cocoa, dry milk, baking powder and salt.
In a large bowl, cream the butter or margarine, brown sugar, and sugar substitute with an electric mixer on medium speed until the mixture is light and fluffy. Add the egg and vanilla; beat for 1 minute. Mix the flour mixture into the creamed mixture, alternately with the coffee. Stir in the nuts. Beat at medium speed until smooth. Pour the batter into a 9 inch square pan that has been sprayed with cooking spray.
Bake at 350 degrees F (175 degrees C) for about 30 minutes, or until the cake pulls away from the sides of the pan. Cool on a wire rack.