Preheat oven to 350.
Whisk together flour, cocoa powder and baking soda. Set aside.
Beat together butter, sugar, vanilla and egg until blended and creamy.
Add dry ingredients to the butter mixture and mix until combined.
Line cookie sheet with parchment paper.
Spread onto prepared pan.
Bake for 8-10 minutes or until top is dry but still soft.
Allow to cool.
Carefully remove from pan.
Use a knife and split sandwich cake in half.
Take frozen ice cream and remove from carton.
Using a knife run under warm water, cut into roughly 3/4" slices.
Arrange ice cream slices on top of half of sandwich cake to cover it completely.
Place other half of sandwich cake on top of ice cream.
Wrap in plastic or parchment and place in freezer for at least 2 hours.
One ice cream sandwiches are firm, remove and cut into 8 rectangles with a sharp knife.