Preheat oven to 350°F. Line baking sheets with parchment.
In a large bowl, cream together butter, brown sugar, and granulated sugar until light and fluffy.
Mix in peanut butter until well combined.
Add eggs, one at a time, mixing well after each.
In another bowl, whisk together flour, salt, baking powder, and baking soda.
Gradually add dry mixture to wet ingredients, mixing until fully incorporated.
Scoop dough (about ⅓ cup each), roll into balls, and roll in granulated sugar.
Place on prepared baking sheets. Flatten slightly using a glass.
Bake for 15 minutes. Open oven, press again gently to desired flatness, and bake for an additional 5–7 minutes or until edges are golden.
Cool completely on wire racks.
Once cooled, top each cookie with a tablespoon of peanut butter, chopped peanut butter cup pieces, and m&m’s.
To make ganache, heat cream in microwave or on stove until just boiling. Pour over chocolate chips in heatproof bowl. Let sit 1–2 minutes, then stir until smooth.
Drizzle ganache over cookies. Let set until chocolate firms up slightly.