Hot Fudge Cake

I made this and thought it was delicious. We also had homemade ice cream which made it even better!
1 c sugar
1/2 c cocoa powder
1 c flour
2 t baking powder
1/2 c milk
4 T melted butter
1 egg yolk
2 t vanilla
1/2 c semisweet chocolate chips
1 c boiling water

Preheat oven to 350 degrees. Spray 8 inch square baking pan with cooking spray. Whisk 1/2 c sugar with 1/4 c cocoa powder and set aside.
Whisk the flour, remaining 1/3 c sugar, remaining 1/4 c cocoa, baking powder in a large bowl. Whisk milk, butter, egg yolk, and vanilla in medium bowl until smooth. stir milk mixture into flour mixture until just combined. Fold in chocolate chips.
Scrape batter into pan. Sprinkle cocoa and sugar mixture on top.Gently pour boiling water over cocoa. Do not stir.
Bake for 24 minutes or until the top of the cake is cracked and the sauce is bubbling. Cool for ten minutes.