Preheat oven to 350 degrees F.

Melt Butter: Add butter to a microwave safe bowl and microwave for about 45 seconds, or until most of it has melted. Allow it to cool for a few minutes.

Make cookie dough: Add melted butter, granulated sugar and brown sugar to a medium bowl and beat with an electric mixer until well blended. Add the egg, egg yolk, and vanilla and mix until combined. Stir in the flour, baking soda, and salt, just until combined. Gently stir in the chocolate chips.

Bake: Press the cookie dough into a 9'' springform pan or lined round cake pan.

Bake for 35-40 minutes. (Cover with a tent of aluminum foil if the top is browning too much). Allow to cool for at least 15 minutes before removing from pan. Cool completely before frosting.

Frosting: In a medium bowl, use an electric hand mixer to mix butter for 30 seconds. Add remaining ingredients and mix for a few minutes, until well-combined and fluffy. Add more milk, to thin, or powdered sugar to thicken, as needed, for a good frosting consistency.

Decorate: Pipe large chocolate swirls around the top outer edge of the cookie (I like the Wilton 1M or 2D piping tips). Add sprinkles.