Position a rack in the centre of your oven and preheat to 350 F. Grease a square baking pan with nonstick spray. Line with parchment paper.
In a large bowl, whisk the sugar and egg until pale and foamy, 1 minute. Add 1/2 cup of the chocolate hazelnut Nutella, oil, milk, vanilla and salt. Whisk until smooth and emulsified. Add the cocoa powder and whisk until smooth.
Add the flour, baking powder, baking soda and whisk until well combined and smooth. Stir in the Frangelico and Creme de Cocoa, then fold in the chopped Lindt chocolate bar.
Pour the batter into the prepared pan, tap the pan gently on the counter to release any air bubbles. Smooth the top with an offset spatula.
Bake the cake until puffed, 40 minutes. Set the pan on a rack to cool for 15 minutes. Then use the parchment paper to lift the cake out of the pan and set on a rack to cool completely.
Using an offset spatula, spread the remaining Nutella over the top of the cake.
Unwrap the Lindor Hazelnut Milk Chocolates. Using a sharp knife, gently slice the truffles in half at their seam. Gently press the Lindor halves into the icing, dome side up. Sprinkle chopped hazelnuts over the cake. Slice and serve.