In a microwave-safe bowl, melt the butter and dark chocolate in short bursts, stirring until smooth.

In a stand mixer fitted with the whisk attachment, beat the eggs, pinch of salt, and brown sugar on medium speed for 6 minutes, until pale and airy. Fold in the melted chocolate mixture gently until combined. Sift in the cocoa, flour, and baking powder, mixing only until smooth. Refrigerate the dough for 40 minutes to firm.

For the caramel, heat the cream in a small saucepan until just below a boil, then set aside. In a wide skillet over high heat, cook the sugar until it begins to melt and caramelize in the center. Lower heat to medium and swirl the pan occasionally until the sugar turns a deep amber. Carefully whisk in the hot cream and salt. Remove from heat, add the butter, and stir until fully incorporated. Let cool slightly.

Preheat the oven to 350°F/175°C.

Roll the chilled dough into even-sized balls. Coat each ball in hazelnut croquant and place on a parchment-lined baking sheet, leaving space between them. Press a small indentation into the center of each ball with your finger or the back of a spoon. Bake for 10–12 minutes.

While still warm, reinforce the indentations with a spoon, then allow cookies to cool completely on a wire rack. Fill each cavity with caramel using a spoon or piping bag.