Steps
1. Preheat your oven to 175°C (350°F) and grease a small baking tin or line it with parchment paper.
2. Melt the butter in a microwave-safe bowl or over a double boiler.
3. Stir in both sugars until combined.
4. Add the egg and vanilla extract, mixing well.
5. Sift together the flour, cocoa powder, espresso powder, and salt.
6. Gently fold the dry ingredients into the wet mixture until just combined.
7. Fold in the chopped dark chocolate or chips if using.
8. Pour the batter into the prepared tin, smoothing the top.
9. Bake for 18-22 minutes until the top is crackly and a toothpick inserted comes out with a few moist crumbs.
10. Let cool before cutting into a single serving brownie.