Preheat oven to 325º F. Prepare a 9-inch baking dish by lining with parchment paper or grease and flour the pan.
In a microwave safe bowl heat butter and chocolate for 45 seconds. Stir well and heat again for 20-30 seconds until it is melted and smooth. Let is cool for 5 minutes.
10 Tablespoons butter, 3 oz milk chocolate
Add granulated sugar and eggs. Beat well until the eggs are completely incorporated and well mixed. Add in heavy cream and vanilla and mix until completely incorporated.
1 1/3 cup granulated sugar, 3 large eggs, 2 Tablespoons heavy cream, 2 teaspoons vanilla extract
Add flour, cocoa powder, baking powder and salt and mix until the flour is just incorporated.
1/2 cup +2 Tablespoons all-purpose flour, 1/2 cup unsweetened cocoa powder, 1/2 teaspoon baking powder, 1/2 teaspoon salt
Pour into a baking pan and sprinkle chocolate chunks over the top.
1/2 cup chocolate chunks
Bake for 35-40 minutes until the edges are completely set, and a tester inserted comes out mostly clean.
Remove from the oven and let cool completely before slicing to serve.