Preheat oven to 375°F. Line 8 wells of a standard 12-cup muffin pan with paper cases.
Melt dark chocolate and chocolate hazelnut spread together in a heatproof bowl over a saucepan of simmering water until smooth. Set aside to cool.
Sift flour, cocoa and baking powder into a large bowl. Add sugar and salt and whisk to blend.
In a separate bowl, whisk together eggs and oil until very smooth. Whisk in milk and vanilla, then stir in cooled chocolate mixture. Make a well in the center of the dry ingredients and pour in wet ingredients. Fold together gently, adding chocolate chips halfway through mixing. Do not over-mix.
Divide mixture between paper cases, filling them almost full. Sprinkle hazelnuts over top and press them down lightly so that they stick. Bake for 18-20 minutes, until skewer comes out clean. Transfer to a wire rack to cool slightly. These are best enjoyed warm!