Preheat oven to 350 degrees. Spray a 9-inch square pan with cooking spray. In a medium bowl, toss together raspberries, 2 tablespoons of sugar and cornstarch. Set aside
In a medium bowl, combine 1/2 cup sugar and 1/4 cup cocoa powder. Set aside.
In a large bowl, whisk together flour, remaining 1/2 cup sugar, remaining 1/4 cup cocoa, baking powder and salt. Whisk in milk, butter, egg yolk and vanilla until smooth (batter will be stiff). Stir in chocolate chips.
Spread batter into prepared pan. Dollop with spoonfuls of raspberries. Sprinkle reserved sugar and cocoa mixture over top.
Carefully pour the boiling water evenly over the cake. Do not stir.
Bake for about 25 minutes, or until the top of the cake is cracked and the edges are firm, but the center is still gooey.
Serve warm with ice cream or whipped cream.