Preheat the oven to 180°C (350°F). Line the baking tray with parchment paper.

Unroll the round puff pastry sheet on the cutting board. Cut it in half into two semicircles.

Whisk the egg yolk with the milk. Brush the mixture over one pastry semicircle.

Sprinkle the brushed pastry with vanilla sugar. Place the second pastry semicircle on top.

Brush the top pastry layer with the remaining egg mixture. Sprinkle it with the remaining vanilla sugar.

Cut the layered pastry into 6 equal triangles. Transfer the triangles to the lined baking tray.

Bake the triangles for 20 minutes at 180°C (350°F). Remove them when puffed and golden brown.

Cool the baked triangles completely at room temperature. Slice each triangle horizontally in half.

Place the whipping cream, 150 g mascarpone, powdered sugar, and Nutella in a mixing bowl. Beat until the mixture begins to thicken.

Add the remaining 100 g mascarpone to the cream. Beat until stiff, smooth, and pipeable.

Transfer most of the Nutella mascarpone cream to the piping bag. Reserve a small portion for decoration.

Pipe cream onto the bottom half of each pastry triangle. Scatter chopped milk chocolate and white chocolate over the cream.

Place the pastry tops back over the filling. Dust the triangles lightly with powdered sugar.

Pipe small cream rosettes over the triangles. Top with chopped Kinder Bueno bar pieces.

Refrigerate the filled triangles for at least 15 minutes. Serve chilled or slightly cool.