• Pre-heat oven to 350 degrees. Line a 9-inch cake pan with parchment paper.
• In a medium bowl, whisk together flour, baking powder, and salt.
• In the bowl of a stand mixer or in a large bowl with a hand mixer, cream together butter and both sugars until light and fluffy.
• Add eggs and vanilla and mix to combine.
• Add the flour mixture to the wet ingredients and mix until just combined.
• Fold in the Reese's Pieces and peanut butter chips.
• Press cookie dough into the prepared pan, as evenly as possible.
• Bake at 350 degrees for 22-25 minutes, until edges are turning golden, but cookie is still nice and gooey in the middle.
• Remove from oven and set on a wire rack to cool. Once cooled, use a knife to loosen edges of cookie and turn out onto a cake plate or platter.
• Put frosting in a pastry bag and pipe along the edges. Add additional Reese's Pieces if desired.

Chocolate Frosting

• In the bowl of a stand mixer or in a large bowl with a hand mixer, beat butter until light and fluffy, occasionally scraping sides of bowl with spatula.
• Mix in cocoa powder and then mix in confectioners' sugar one cup at a time, until well-blended.
• Mix in milk, 1 tablespoon at a time, only adding in second tablespoon if necessary.