Preheat the oven to 180c and grease a 20cm square baking tin and line with baking paper.
Melt the butter and dark chocolate together in a bowl over hot water.
Pour 275g of golden caster sugar into a bowl and add 3 large eggs. With an electric mixer on maximum speed, whisk the eggs and sugar until they look thick and creamy, like a milkshake! This can take about 4-5 minutes. TIP, you'll know it's ready when the mixture becomes pale and about double it's original volume. Add 2 tablespoons of the salted caramel sauce to the mixture.
Pour the chocolate mixture over the eggy mousse, then gently fold together.
Hold a sieve over the bowl of eggy chocolate mixture and sift the cocoa powder and flour mixture, shaking the sieve from side to side, to cover the top evenly. Gently, fold in the mixture.
Pour a ? into the baking tin and using a teaspoon carefully add thick lines (vertically) of the rest of the salted caramel sauce (about 8 lines), then scatter about ? of the pouch of popcorn on top spread out over the sauce. Then pour the rest of the mixture on top and smooth out.
Put in the oven and bake for 25-30 mins. The brownies should be nice and golden on top. Press in extra pieces of popcorn evenly spread out into the top once it is cooked.
Leave to cool for several hours and cut into 9 squares. Enjoy these delicious gooey brownies with a large scoop of vanilla ice-cream. Finish the rest of the pouch of popcorn too of course!