Heat your oven to 160°C Fan/180°C/355°F/Gas Mark 4.
Grease your baking tin and then cover with greaseproof paper. To do this cut a piece of paper that will cover the long sides and base of the tin. You want to ensure the paper overhangs the edges of the tin so that you can hold it to remove your brownies when cooled. The shorter sides will be fine if just greased as long as your tin is in a good condition.
Add boiling water to your saucepan, it needs to be about a third full. Keep to a gentle simmer on a low setting and place your heatproof bowl on top. Be careful, it will be hot!. The base of the bowl should not touch the water. This is known as a Bain-Marie.
Add your butter to your heatproof bowl and gently melt.
Mix in 200 grams of your chopped dark chocolate and stir until melted.
Carefully remove your heatproof bowl and pour the mixture into your large bowl, then mix in your caster sugar.
Leave to cool until at room temperature.
Sift your flour, cocoa and baking powder together and then fold into your cooled chocolate mixture. Folding means turning the mixture over onto itself, rather than mixing in circles, this retains air which would be lost if mixing.
Beat your eggs and then fold them into your brownie batter.
Add half of your brownie batter to your prepared tin.
Place over a layer of After Eights.
Cover with the rest of the brownie batter.
Finally, add another 6 or so After Eights to the top of the batter and dot over the remaining 25 grams of chopped chocolate. Don't press the chocolate into the mixture, just leave on the top to melt as it bakes.
Place in the centre of your oven and bake for 30-35 minutes.
Your After Eight brownies are ready when there is a slight wobble in the centre. A skewer inserted into the centre should not come out clean, like with cake and it should also not come out with lots of wet mixture stuck to it either. The skewer should just have a few cakey fudgy bits on it, ie something in between!
Leave to cool completely in your brownie tin, then cut up into squares.