Preheat the oven to 350F (180C or 170C fan forced). Line an 8x8 inch (20x20 cm) square baking pan with baking paper, leaving an overhang around the edges. Or lightly grease.
In a large bowl add the chopped butter and finely chopped chocolate (or chocolate chips). Melt together in a double boiler over low heat or in the microwave. If microwaving, using 45-second intervals on medium power (not high power, which is often the default) and stir between each interval until smooth. Melting slowly on a low heat is key to ensure the chocolate doesn't go grainy.
Remove from the heat and whisk in the sugar.
Then whisk in the eggs and vanilla extract.
Carefully stir in the flour, cocoa and salt. If the cocoa is lumpy, I recommend sifting it first.
Spoon about ½ of the brownie batter into the prepared pan and smooth into an even layer.
Place the Aero Bars on top to create an even layer. Ensure that the chocolate bars are pressed slightly into the brownie batter. I had to shave a little off the tops of each bar so that they would fit in the pan. You may need to break up/cut each bar a bit, depending on which size of Aero you're using.
Drop spoonfulls of the rest of the batter on top of the chocolate bars.
Then spread into an even layer.
Bake in the preheated oven for about 28-33 minutes or until the top looks set and an inserted toothpick comes out with a few damp crumbs but no gooey brownie batter.
Cool the brownies in the pan until the pan is no longer warm to the touch. This will take a few hours.
When ready to slice, lift the brownies out of the pan using the overhang of the baking paper and place on a cutting board. Slice with a sharp knife. If you greased the pan, slice the brownies in the pan.