• Preheat oven to 350 F. Spray a 9" spring form pan, cake pan or pie plate with cooking spray. Set aside.

To make the cookie cake

• Place the butter and sugars in a stand mixer fitted with a paddle attachment. Beat on medium-high speed until light and fluffy, about 4 minutes. Add egg, yolk and vanilla extract and mix on low until smooth, about 30 seconds.
• Slowly mix baking soda, salt, cornstarch and flour, being careful not to over mix. I usually stop when I still have some flour remnants around the bowl. Fold in the chocolate chips.
• Spread cookie dough evenly into baking pan and place a few extra chocolate chips into the top.
• Bake for 22-27 minutes or until very lightly browned on top. You want the cookie cake to be extra soft in the center, so be careful not to over bake. Allow the cookie cake to cool completely before frosting and cutting.

To make the buttercream

• In a large mixing bowl, beat the butter until light and creamy. Add cocoa powder, powdered sugar, milk, vanilla and a sprinkle of salt. Beat on high for 2-3 minutes. Pipe the buttercream using a piping tip or you can even just cut a small corner on a ziplock bag and do little dots. Sprinkle with a little salt and some chocolate jimmie sprinkles.