Preheat the oven to 175°C (350°F). Lightly grease a 4-inch round cake pan and line the bottom with parchment paper so the cookie cake comes out easily.

In a large mixing bowl, beat the softened butter with the light brown sugar and granulated sugar until the mixture is smooth and creamy. Add the egg, egg yolk, and vanilla extract, then mix until everything is well combined.

In another bowl, combine the flour, baking soda, and salt. Slowly add the dry ingredients to the butter mixture, stirring until just combined. Pour in the milk and give it a quick mix. Gently fold in the 40 g of chocolate chips, being careful not to overmix the dough.

Press the dough evenly into the prepared cake pan. Sprinkle the mini chocolate chips over the top. Bake for 15–18 minutes, until the edges are lightly golden and the center is set but still a little soft.

Let the cookie cake cool in the pan for about 10 minutes before carefully transferring it to a serving plate. Allow it to cool completely. If you like, spread chocolate frosting over the top and add some colorful sprinkles. Cut into 6 slices and enjoy.