Heat oven to 350°F. Grease an 8 or 9-inch baking pan with cooking spray.
Stir together sugar, flour, cocoa, baking powder, baking soda and salt in large bowl.
Add sour cream, milk, oil and vanilla; beat on medium speed for 1 minute.
Stir in boiling water (batter will be thin). Pour batter into prepared pan.
Bake for about 15 minutes, or until a toothpick inserted in the center comes out clean.
Allow cake to cool completely.
Spread a thin layer of fudge sauce over the top of the cake.
Remove ice cream from the freezer and allow to sit for 10-15 minutes. Stir well to soften enough to spread onto the cake.
Freeze for several hours, until ice cream is firm.
Beat whipped cream for 1 minute. Mix in powdered sugar and vanilla and beat until stiff peaks. Smooth whipped cream over the top and top with sprinkles, maraschino cherry, or other crushed candies for decoration, if desired.
Store Ice cream cake in the freezer, covered well, without whipped cream topping for 2-3 months of for 1 week with whipped cream topping.
Add different toppings: top the cake with frosting, sprinkles, crushed cookies, mini chocolate chips, M&M's, crushed Reese's or other small candies.
Oreo Ice Cream Cake: substitute Oreo crust for the chocolate cake. Top with ice cream, hot fudge, and whipped cream. Freeze.
Make Ice cream cupcakes: Bake cupcakes and allow them to cool completely. Use a small knife to cut out a hole in the top of the cupcakes, scooping out some of the center. Fill it with ice cream and smooth fudge sauce on top of the cupcake or dip it in melted chocolate. Freeze.
Cookie Ice Cream Cake: substitute Chocolate Chip Cookie Bars in place of cake. Allow them to cool completely and follow steps 7-10 of the recipe card.
Brownie Ice Cream Cake: Substitute brownies or blondies in place of cake. Allow them to cool completely and follow steps 7-10 of the recipe card.