Preheat oven to 350 degrees F. Grease a 9- x 13-inch baking pan.
In a medium saucepan over low heat (or in the microwave), melt the chocolate and butter together. Mix in sugar, eggs, vanilla and Kahlua. Stir in flour, baking powder and salt.
Spread in prepared pan. Bake at 350 degrees F, 25 to 30 minutes or until batter starts to pull away from sides of pan. Cool.
For the frosting:
In a small bowl, mix Kahlua with instant coffee, stirring until coffee is dissolved. In a medium bowl, cream butter, powdered sugar and vanilla together. Add coffee mixture and beat until well blended. Add milk to desired consistency. Spread frosting over cooled cake.
TOPPING:
In a small saucepan, melt chocolate and butter together. Cool slightly, then drizzle topping over frosting. Cool until chocolate is set.
How to serve the cake:
Cut into squares and serve with vanilla ice cream.
How to freeze the cake:
Make the entire recipe; cool. Cut into pieces. Freeze the pieces already cut into small squares. Defrost and serve!