Preheat oven to 180 degrees Celsius Gas Mark 4.
Grease and line a 20 x 30cm, deep baking dish.
For the cookie layer:
Beat the butter, brown sugar, sugar and vanilla extract together in a large bowl until creamy. Add the egg; beat until light and creamy for about 2 minutes.
In another clean bowl, whisk together sieved flour, salt and bicarbonate of soda. Gradually stir flour mixture into the butter mixture until the dough is combined.
Stir the roughly chopped Butlers 40% milk chocolate into the dough.
Spread the dough into the bottom of the prepared baking dish and cover completely.
For the brownie layer:
Melt the butter and chocolate on a low heat.
Hand whisk the sugar and eggs until well combined and pale in colour. When the chocolate mixture has cooled, add to the sugar and egg mixture and stir together.
Sift the flour into the bowl and then mix all of the ingredients together. Pour over the cookie base.
Bake in the preheated oven for about 30 to 35 minutes, until a skewer inserted into the centre of the Brookies mixture comes out clean.
Cool completely before cutting into squares.