Preheat the oven to 350°F or 175°C and line an 8×8 inch pan with parchment paper.
Whisk melted butter with both sugars, then mix in eggs and vanilla.
Fold in flour, cocoa powder, and salt just until combined.
Stir the chocolate fudge sauce with peppermint extract until smooth.
Layer half the batter, the mint fudge filling, then the remaining batter.
Bake for 30 to 35 minutes until edges are set and center is slightly soft.
Cool completely, add toppings if desired, and slice.