Preheat the oven to 190°C (374°F) using the convection setting.
Beat the butter and sugar together on medium speed for 2-3 minutes until light and fluffy. Add the egg yolks one at a time, beating well after each addition. Gently fold in the melted dark couverture. In another bowl, combine the flour, grated hazelnuts, and 2 tablespoons cocoa powder. Fold the dry mixture into the wet mixture until just combined.
Spread about half of the batter into a greased brownie pan, ensuring the base is completely covered.
Arrange the Toffifee candies on the batter in a grid pattern with the chocolate side facing down, spacing them about 1 cm apart. Carefully spread the remaining batter over the Toffifees without moving them. Smooth the top. Bake for 15 minutes.
While the brownies bake, combine the remaining sugar and a small amount of water in a saucepan over low heat. Use a wet pastry brush to wipe water along the inner walls of the pot. Heat without stirring until the mixture reaches approximately 125°C (257°F).
Beat the egg whites with a pinch of salt in a clean bowl until stiff peaks form. With the mixer running, slowly pour the hot sugar syrup into the egg whites in a thin, steady stream. Continue beating until the meringue cools to room temperature.
Spread or pipe the meringue onto the baked brownie layer using a spatula or pastry bag.
Reduce the oven temperature to 160°C (320°F) and bake for 8-10 minutes until the meringue is lightly golden. Allow to cool completely. For easier cutting, briefly freeze if needed before slicing into squares aligned with the Toffifee positions. The dessert will be moist.
Alternative: Omit the sugar syrup and water and use plain meringue instead by beating egg whites with a pinch of salt until stiff peaks form. This will be less creamy than the cooked meringue version.
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