Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4 and grease and line your chosen tin
Melt the dark chocolate and butter together in a pan on a low heat. When fully melted, stir together and remove from the heat. Let it cool for 10 minutes
In a mixing bowl use an electric mixer to whisk together the eggs, light brown sugar and vanilla extract for 5 minutes until thicker and paler in colour
Add the chocolate mixture to the egg mixture, whisking constantly as you pour it in
Add the plain flour and whisk in gently with a hand whisk until you can't see any more flour
Pour the batter into your lined tin. Bake for 15-20 minutes if using a 9" x 12" traybake tin, 25-30 minutes if using a 9" square tin, or 35-40 minutes if using a 8" square tin
Let the brownies cool completely in the tin before removing and slicing up
Store in an airtight container in a cool place and eat within 3 days