First set out butter, milk, and one large egg. We want the butter to soften and the cold ingredients to reach room temp. before baking. Measure out all ingredients in advance for fuss-free baking.
Pre-heat oven to 350°F.
First make the cookie cake. In a large bowl whip softened butter, then cream together with brown and granulated sugar with an electric hand mixer until well-mixed and creamy in consistency.
Add in the egg and vanilla and continue to beat with the mixer to incorporate.
In a separate bowl, mix together flour, cornstarch, baking soda, and salt.
Add flour mixture to the larger bowl slowly while you continue to beat on low.
Once your dough is mixed, fold in chocolate chips, saving a spoonful or two for topping the cake at the end.
Trace your heart-shaped cookie cake pan onto parchment paper to make your liner. Place parchment paper in the pan then press in your dough.
Top dough with remaining chocolate chips. Save adding the M+M's for after the cake has baked to prevent cracking.
Bake on the center rack for 14-16 minutes. Mine was done in 15 minute but oven temperments tend to vary so be sure to check on it at around 13-14 minutes.
Once cookie is ready, use an oven mitt to remove pan from oven and set on top of a wire baking rack. Add M+M's immediatley while the cookie is still hot by pressing into the cake in any pattern you choose.
Allow cookie cake to completley cool before icing and slicing. Cake will be very soft in the center and set once it has cooled.
TO MAKE THE FROSTING
First, in a small saucepan over medium heat, melt the chocolate and the butter, stirring often and mixing well to emulsify. Allow it to cool for 15 minutes.
Next in a medium-large bowl with your electric hand mixer, whip together chocolate/butter mixture with vanilla extract and the teenist bit of salt to enhance the chocolate flavor. Blendin half the milk/cream then sprinkle in powdered sugar slowly in 3-4 batches, while mixing until fluffy and fully incorporated.
Add additonal milk/cream to thin the frosting as needed to reach desired consistency. To make it creamy enough to pipe, I needed an additonal 2 TBSP of cream for 4 TBPS total. Add to a piping bag with a large star tip (or your favoite piping tip) and decorarte cookie once cooled. Feel free to add festive sprinkles on top!
Notes
Nutrition Facts below are estimated for 12 servings using an online recipe nutrition calculator. Adjust as needed based on slice size chosen (plus any swaps/extras) and enjoy!