Preheat the oven to 160˚C/fan 140˚C/gas mark 3. Grease an 8x8in baking tray with butter, then line with foil.
Break up the chocolate and put in a medium-sized bowl with the butter. Place the bowl over a pot of gently simmering water, making sure the water doesn’t touch the bottom of the bowl. Stir occasionally until completely melted and combined.
Whisk together the sugar and eggs until they have become smooth and creamy. Add the chocolate mixture and combine thoroughly. Slowly sift in the flour, stirring until completely combined. Add the cocoa powder, a pinch of salt and Maltesers.
Pour the mixture into a foil-lined tin. Smooth the top and bake in the middle of the oven for 20-25 minutes, until the top is cooked and cracking slightly but the inside is still gooey. Remove the brownies from the oven and leave to cool on a wire rack. Once completely cooled, take out of the tray, remove the foil and chop into squares.
Place the sugar and honey in a large saucepan with 120ml water and stir continuously over a medium heat until all the sugar has dissolved. Lower the heat and leave for 10 minutes, until it is gently simmering and has started to turn a light caramel colour.
Remove from the heat and whisk in the cream. Once combined, gradually add the butter, vanilla extract and ½ tsp salt. Return to the low heat and stir for 2 minutes to allow the caramel to thicken.
To serve, pour the caramel sauce over the brownies and sprinkle over the chopped Maltesers and sea salt flakes.