Meanwhile, make the brownie. Melt the butter and dark chocolate in a heatproof bowl in a microwave oven on medium in 20-second bursts for about 1½ minutes, stirring after every burst until melted, or place the bowl over a small pan of simmering water (make sure the bowl doesn’t touch the water) and stir until melted. Set aside to cool slightly.
Put the eggs and caster sugar into a separate bowl and whisk in a stand mixer or with an electric whisk for 5 minutes or until the mixture is thick, pale and has doubled in size.
Gently fold the cooled melted chocolate mix into the egg mixture until combined. Fold in the flour, cocoa powder and white chocolate chunks. Pour into your flapjack case, sprinkle with the sea salt, then bake for 22–28 minutes or until it has a little wobble in the centre but is set around the edges.
Cool completely in the tin then you can refrigerate it overnight to make perfect slices, if you like. Serve in slices. Keep refrigerated in an airtight container for up to 4 days.
Tip: If you’ve refrigerated the tart overnight, either leave it out at room temperature for an hour or loosen around the edge with a blow torch to help remove it from the tin.
Adapted from I’ll Bake (Pavilion)
