Chocolate Shortbread Cookies

Preheat the oven to 300 degrees F (150 degrees C).

Mix flour and cocoa powder together in a bowl.

Beat butter in a large mixing bowl with an electric mixer until creamy. Add sugar and vanilla extract; mix well. Mix in flour mixture gradually, 1/2 cup at a time, until incorporated; dough will resemble damp sand.

Press dough firmly into a 9x13-inch baking pan. Cut into 1x2-inch rectangles. Prick each rectangle a few times lightly with a fork.

Bake in the preheated oven until top looks dry and edges start to pull away from the sides of the pan, 20 to 25 minutes. Let cool before lifting out of the pan.