Preheat the oven to 350°F. Lightly coat a 6-inch baking dish with cooking spray.
Place 2 tablespoons of butter in a small microwave-safe mixing bowl. Melt it in the microwave for 30 seconds, stir and warm it again for 10 to 20 seconds or until melted.
Add ¼ cup brown sugar and 1 tablespoon of granulated sugar to the melted butter and stir. Mix in the egg yolk and vanilla extract.
Add 1/3 cup of flour and 1/4 teaspoon baking powder. Stir until the dough is well combined. Mix in almost all of the chocolate chunks, reserving a few bigger pieces for the top of the cookie.
Transfer the cookie dough to the prepared baking dish. Lightly press it into the dish and then press the remaining chocolate chunks on top.
Bake the cookie for 20 to 23 minutes or until it’s lightly golden on top and beginning to pull away from the sides of the baking dish.
Cool the cookie for five minutes before serving it warm with vanilla ice cream.
Notes
If you are using unsalted butter, add a pinch of salt when you add the flour.I use chopped dark chocolate or you can try milk chocolate. You can also use chocolate chips.I recommend checking the cookie once it's baked for 20 minutes. You want it to be just set in the middle and starting to pull away from the sides of the pan. If it hasn't reached this stage, keep baking it checking every minute or so until it's done. I like to let it cool for a few minutes in the pan.You can prepare the cookie dough ahead, press it into the pan, and refrigerate it until you're ready to bake the cookie. Let it warm up at room temperature for 10 minutes or so before baking it.