Butter and line an 8-inch square cake tin with baking parchment. Tip the 140g butter and dark chocolate into a heatproof bowl. Set it over a pan of just simmering water (without letting the bowl touch the water) and let it melt, stirring regularly. Add a large pinch of flaky salt and remove from the heat to cool slightly.

Heat the oven to 190C/170C fan/gas 5. Whisk the whole egg and egg yolks, sugars and vanilla extract in a stand mixer or in a bowl using an electric whisk for 4-5 mins, until pale, thickened and forming ribbons.

Cut the Creme Eggs® in half vertically and scoop the fondant centre into a small bowl. Roughly chop the remaining chocolate shells into chunks and set aside. Tip the melted butter and chocolate mixture into the sugar and eggs. Whisk again briefly, scraping the sides down to ensure everything is fully combined.

Sift the remaining dry ingredients into the wet mixture and gently fold until no flour streaks remain. Finally, add the chopped Creme Egg® chocolate chunks and stir to distribute evenly. Pour the batter into the prepared tin and spread evenly. Use a teaspoon to dot the fondant filling across the top of the brownie. Then use a cocktail stick or metal skewer to swirl the top. Bake on the middle shelf of the oven for 22-25 mins, until the top is crackled but there is still a slight wobble when shaken. Allow to cool, then chill for at least 1 hr before cutting into squares.