Preheat oven to 350°F (175°C). Line an 8-inch (20cm) square tin with parchment paper.

In a large bowl, whisk together melted butter and sugar until smooth. Add eggs and vanilla and beat until glossy.

Sift in the flour, cocoa powder, and salt. Gently fold until just combined - don’t overmix.

Stir in all three types of chocolate chips, saving a small handful to sprinkle on top.

Pour the batter into the prepared tin and spread evenly. Sprinkle over the reserved chocolate chips.

Bake for 30–35 minutes, until the edges are set but the centre is still slightly gooey. A skewer should come out with a few moist crumbs.

Allow to cool completely in the tin before slicing into squares.