Heat oven to 350 degrees F and grease a 9" round cake pan.

Set pan aside.

Add butter, 2/3 cup of semisweet chocolate chips and 1/3 cup of peanut butter chips to a microwave-safe bowl and heat in 20-second intervals on half-power in the microwave, scraping down the sides and stirring after each interval until the mixture is smooth and melted.  You may also do this step on the stove with a double boiler if you prefer.

Once melted, add butter mixture to a mixing bowl and add sugar.

Whisk until sugar is thoroughly combined.

Add eggs and vanilla extract and whisk again until all mixed in.

Add flour and salt and stir until the dry ingredients are just mixed in. The batter will be fairly thick and smooth.

Stir in the chopped Reese's peanut butter cups, 1/4 cup of semisweet chocolate chips and 1/4 cup of peanut butter chips.

Pour batter into the prepared cake pan and smooth the top with a spoon so that it's level.

Press additional chips and/or chopped Reese's peanut butter cups into the top if you like.

Bake for 22-28 minutes, or until the top has set, the edges have begun to brown, and a tester inserted into the center comes out clean.

Remove from oven and let brownie cake cool completely in the pan before slicing.

To decorate, melt semisweet and peanut butter chips in the microwave and transfer to two separate piping bags.

Pipe zigzags or whatever design you please over top.

Once cooled, decorated, and ready to slice, run a knife around the outside edge of the brownie cake to loosen it, and cut into slices.