Preheat your oven to 350°F. Line an 8x8-inch square baking pan with parchment paper, leaving a 2-inch overhang on each end and butter the exposed sides.
First make the peppermint bark. Place chopped chocolate in a heatproof bowl set over a saucepan of simmering water and stir frequently until mostly melted and smooth. Remove from heat and continue stirring until the last of the chocolate has melted. Spread it out over a piece of parchment paper and sprinkle crushed candy cane on top. Place it in the fridge to set for 20 minutes.
For the brownie batter, combine chocolate and butter in a medium bowl over a saucepot with ½-inch of simmering water and stir together until completely melted, smooth and glossy.
Combine eggs with sugar, vanilla and peppermint extract (if you choose to use it) in a large bowl and beat with a mixer or a whisk until pale, thick and fluffy. Mix in salt and espresso. Stir in warm chocolate mixture until evenly blended and smooth.
Sprinkle the flour over the chocolate mixture and stir it in until evenly incorporated. Pour batter into prepared pan and smooth out the surface. Break up the chocolate bark into large pieces and place them on top.
Bake for 25-30 minutes until slightly puffed and a toothpick inserted into the center comes out with a few moist crumbs attached. Transfer pan to a wire rack and let cool completely.