Preheat the oven to 350°F (175°C). Line 8x8in (20x20cm) baking dish with parchment paper.
Chop the snickers into small pieces. Reserve about 1/3 of candy for topping. And toss the remainder of the snickers bites with a tablespoon of flour.
In a medium heatproof bowl, melt the chocolate and butter over simmering water until smooth.
Remove from heat, stir in the sugar. Cool slightly.
Add the eggs and vanilla extract and mix until well combined.
Add the flour, espresso powder, baking powder and salt. Mix just until you see a little bit of flour and stir in the floured snickers.
Pour the mixture into the prepared baking dish. Bake for 18 minutes. Sprinkle the reserved chopped snickers over the brownie. Bake for another 4 minutes.
Cool completely on wire rack before removing from the pan. Cut into squares and serve.