Preheat oven to 350 degrees. Spray an 8 or 9-inch baking pan with cooking spray.
In a medium bowl, whisk together 1/2 cup sugar and 1/4 cup cocoa powder. Set aside.
In a large bowl, whisk together the flour, remaining 1/2 cup sugar, remaining 1/4 cup cocoa, baking powder, and salt.
In a medium bowl, whisk milk, butter, egg yolk, and vanilla until smooth. Stir milk mixture into flour mixture (batter will be stiff) Fold in chocolate chips.
Spread batter into prepared pan. Sprinkle reserved cocoa and sugar mixture evenly over the top.
Gently pour boiling water over batter. Do not stir.
Carefully place in oven and bake until the top of the cake is cracked and the edges are firm, but the center is still gooey, about 23-25 minutes.
Cool on a wire rack for at least 10 minutes. Serve warm topped with vanilla ice cream.