Pre heat the oven to 175 C | 160 Fan | Gas 5 and line a 20cm square tin with baking paper.
Pop the dark chocolate, butter, vanilla extract and coffee granules into a large pan and place over a low heat on the hob (lowest possible setting). Stir constantly until the chocolate has nearly melted, then remove from the heat and continue to stir until completely melted. Make sure you scrape the bottom of the pan and sides when stirring to stop the chocolate from burning. Do not rush this stage.
Stir in the sugar until well combined.
Stir in the eggs and additional yolk one at a time until combined. Do not beat in the eggs, we don't want to aerate the mixture.
Fold in the flour and cocoa powder until just combined.
Scrape the brownie batter out of the pan into the prepared square tin and use a spatula to level out the mixture. Place in the centre of the oven and bake for 25-30 minutes. Test the brownie by poking a tooth pick in the centre of the brownie. Unlike a cake the tooth pick should have some brownie crumbs stuck to it. If the tooth pick comes out with runny brownie batter on it pop it back in the over for a little longer. It's always better to slightly under bake a brownie than over bake.
When the brownie is ready remove it from the oven. Allow the brownie to cool in the tin for 5-10 minutes. Then carefully remove it from the tin and place on a cooling rack to cool to room temperature.
When the brownie has cooled to room temperature break the white chocolate into pieces. Pop 100g in a heatproof bowl and 50g in two other heatproof bowls. Heat the 100g chocolate over a bain-marie (see link below) and stir constantly until completly melted. Remove from the heat and repeat with the two 50g bowls of chocolate. When all the chocolate has been melted stir in 2-3 drops of red food colouring to one of the 50g bowls. Then stir 2-3 drops of blue food colouring to the other 50g bowl. Don't add too much food colouring, we want nice light pastel shades. You should now have one bowl with 100g melted white chocolate, one bowl with 50g melted pink chocolate and another bowl with 50g melted blue chocolate.
Spoon all of the melted white chocolate into the centre of the brownie. Use the back of a spoon to spread the chocolate over the brownie. I like to leave a little bit of brownie uncovered around the edges but you can cover it all if you like. No need to be neat with this, the messier it is the better it looks.
Use a teaspoon to drizzle the pink chocolate over the brownie, then repeat with blue chocolate. I zigzag across the bownie and back again.
Use a rolling pin to roughly smash up the mini eggs and scatter over the brownie. Leave to chocolate to set for an hour or two before cutting into 16 equal pieces. If you can't wait, you can tuck straight in.
Keeps for 5 days in an airtight container. Like they will last that long!