Pre-heat oven: 180°C / 160°C (fan) / Gas 4 / 350°F

In a heatproof bowl, melt the butter and dark chocolate either in the microwave (30secs at a time) or over a double boiler, stirring until smooth.

In a separate mixing bowl, using an electric mixer beat the eggs and sugar. This should whip up to thick and creamy like a milkshake and can take roughly 3-8mins.

Pour the cooled chocolate mixture over the eggy mousse, add in the vanilla extract and then gently fold together.

In a separate bowl, sieve the plain flour and cocoa powder, whisking together and stirring in the chocolate chips.

Add the wet ingredients into the dry mixture, slowly folding in until only just combined.

Pour the brownie batter into a greased and lined 8inch baking pan.

Bake for 30-35mins, until papery and cracked on top, but with a slight wobble in the middle.

If using, melt the dark chocolate for the drizzle slowly in a microwave (15secs at a time then stir) or using a double-boiler.

Using a piping bag with a small round piping tip, a sandwich bag with the corner cut off or just a teaspoon to drizzle the melted chocolate over the top of your cooled brownies.

Repeat the above two steps with the milk chocolate then the white chocolate.