Preheat the oven to 350°F and line a small square baking pan with parchment paper.
Place the chopped dark chocolate and butter in a saucepan over low heat and stir until fully melted and smooth.
Remove from heat and whisk in the espresso powder and sugar until combined.
Stir in the egg and vanilla extract until the mixture looks glossy.
Fold in the flour and salt until no dry streaks remain.
Spread the batter evenly in the prepared pan and bake for 20 to 25 minutes until the edges look set.
While the brownies bake, melt the dark chocolate chips and pipe or spread the chocolate into small bat wing shapes on a sheet of parchment. Let them harden at room temperature.
Cool the brownies completely in the pan, then cut into squares and place one or two chocolate bat wings on each piece before serving.