Pre-heat your oven to 160C Fan/350F/Gas Mark 4 and grease and line a 9" square tin (see notes if you have a different sized tin)
Melt the dark chocolate and butter together in a pan on a low heat. Stir together until fully melted, remove from the heat. Let it cool for 5-10 minutes
In a mixing bowl use an electric mixer to whisk together the eggs, light brown soft 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 or spatula until you can't see any more flour
Gently mix in the milk chocolate chips and the chopped up Custard Creams. Pour the batter into your lined tin
Sprinkle over the chopped up Custard Creams for the topping
Bake for 25-30 minutes, or until a cocktail stick poked into the centre comes out with moist crumbs on it, but is not wet. Let the brownies cool completely in the tin, then chill for 2 hours in the fridge, before removing and slicing up
Store in an airtight container in a cool place and eat within 3 days
Notes
Bake for 20-25 minutes if using a 9" x 12" traybake tin, 25-30 minutes if using a 9" square tin, or 30-35 minutes if using a 8" square tin. Test they are done by poking in the centre with a cocktail stick. It should come out with moist crumb on it, but not be wet. I wouldn't recommend using anything smaller than an 8" square tin.
Make sure your eggs are at room temperature before you start baking. The butter can be cold for this recipe, as you are going to melt it anyway. It will just take slightly longer to melt.
I used unsalted butter for this recipe.
I used light brown soft sugar for the brownies, which I do recommend using as it makes them extra fudgey, but you can also use caster sugar.
For all of the answers to more questions about brownies, check out my Chocolate Brownies guide!
Although I provide cup measurements, I highly recommend weighing your ingredients out using digital kitchen scales. It is the most accurate way to measure ingredients and will ensure the best results. Digital scales are very low cost and can be purchased for around £12 ($16.50).
For teaspoon (tsp) and tablespoon (tbsp) measurements, please use measuring spoons and not the type of spoons you eat with. Again this will ensure accuracy and provide the best results.