Pre-heat your oven to 160 degrees C.
Line an 8" square cake tin with baking paper.
Place the butter in a saucepan over a low heat on the hob and let it melt gently. Don't let it boil. Once it is completely melted, remove from the heat.
Break the chocolate into squares and add to the melted butter, Leave to warm up a little, then stir it all together. The heat from the melted butter, will melt the chocolate. If it needs a little hand, put the saucepan back on the hob with a very gentle heat, stirring all the time, until the chocolate has completely melted. (See note below to melt in a microwave.)
Place your eggs in a large mixing bowl, or stand mixer if you have one. Whisk until thick. This is the stage that gets air into the mixture to help the brownies rise.
Once the eggs are thick and pale, add the light brown sugar and beat for another two minutes until combined.
Mix the cocoa powder into the slightly cooled chocolate and butter mixture.
Pour the chocolate mix into the eggs and fold together until well combined.
Pour into the lined cake tin and bake in the oven for 25-30 minutes. 28 minutes is the magic number for me. The bake needs to be a little wobbly in the middle when you pull it out of the oven. The top will have lost its shine, and cracked a little.
Let the brownie cool in the cake tin. To make it easier to slice, pop it in the fridge once its completely cooled and slice when chilled.