Melt the Chocolate: Place the milk chocolate and the white chocolate in two separate heatproof bowls. Melt them gently using a double boiler or the microwave. If using a microwave, heat in 20-second bursts, stirring between each burst to prevent scorching.

Prepare the Cream: In a small saucepan, warm the heavy cream over medium heat. Do not let it boil. You want it hot enough to melt the chocolate, but not so hot that it splits. Once steaming, divide the cream evenly between the two bowls of melted chocolate.

Stir Until Smooth: Whisk the white chocolate mixture until it is glossy and uniform. Do the same with the milk chocolate mixture. The heat from the cream should create a silky ganache texture.

Add Hazelnut Spread: Stir the two tablespoons of hazelnut spread into the warm milk chocolate mixture. The residual heat will melt the spread, allowing it to incorporate fully into the chocolate.

Combine and Swirl: Pour both chocolate mixtures into a shallow serving dish or a lined square baking pan. Use a spoon or a skewer to gently swirl them together. Do not overmix; you want distinct ribbons of white and brown chocolate.

Add Toppings: Immediately top the surface with the chocolate hazelnut truffles, chocolate chips, and crushed hazelnuts. Press the toppings down very lightly so they stick to the surface as the ganache sets.

Chill: Place the dish in the refrigerator uncovered for two to three hours. The dessert is ready when the center is firm to the touch and no longer jiggles.

Serve: Slice into squares or spoon into bowls. Serve chilled for the cleanest slices and the richest flavor.