1. Preheat the Oven:

1. Preheat your oven to 350°F (175°C).
2. Grease a 9-inch round cake pan or line it with parchment paper.

2. Mix the Dry Ingredients:

1. In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

3. Cream the Butter and Sugars:

1. In a large mixing bowl, use an electric mixer to beat the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2–3 minutes.
2. Add the egg, egg yolk, and vanilla extract, and mix until smooth.

4. Combine Wet and Dry Ingredients:

1. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
2. Fold in the chocolate chips (and sprinkles or M&M’s, if using).

5. Prepare for Baking:

1. Press the dough evenly into the prepared cake pan. Smooth the surface with a spatula.
2. If desired, press a few extra chocolate chips or sprinkles onto the top for decoration.

6. Bake the Cookie Cake:

1. Bake for 20–25 minutes, or until the edges are golden and the center looks set. Be careful not to overbake—it’s better for the center to remain slightly soft.
2. Let the cookie cake cool completely in the pan on a wire rack.

7. Decorate (Optional):

1. Once cooled, decorate the cookie cake with frosting, more sprinkles, or piped designs around the edges.

8. Serve:

1. Slice the cookie cake like a regular cake and serve with a scoop of ice cream or a glass of milk!