Preheat oven to 175 degrees C. Grease 3 large baking trays. Cut three 25cm rounds of baking paper and set each round on each baking tray.
Beat butter and the brown and caster sugars together in a bowl using an electric mixer until fluffy and smooth. Mix eggs, 2 teaspoons vanilla extract, bicarb and salt into creamed butter until just combined; stir in flour until dough is just combined. Fold chocolate chips into dough.
Divide dough into 3 parts; press 1 part onto each baking paper round.
Bake in the preheated oven until cookie rounds are browned and crisp around the edges and set in the middle, 15 to 20 minutes. Remove from oven and cool completely.
For the Frosting
Stir melted butter, milk and vanilla extract together in a bowl. Beat icing sugar and cocoa powder together in a separate bowl until evenly combined. Beat butter mixture into cocoa mixture until frosting is smooth and thickened.
Peel baking paper off the bottom of each cookie round. Set aside the prettiest round to be used for the top layer. Spread 1/2 the frosting onto 1 cookie round and top with a second cookie round. Spread remaining frosting onto second cookie round and top with the third cookie round.
Be sure to let the cookies cool completely. I like to make them the day before.