Mix together the eggs, milk, sugar, cocoa powder, vanilla and espresso powder. The best way to get the cocoa powder completely incorporated is to use a handheld electric mixer, and mix on medium for a full minute.
Transfer to a shallow baking dish and set aside.
Using a paring knife, carefully cut a pocket into the bread without poking the knife through the opposite side. I found it helpful to put the knife back in a second time and gently work it along the length of the bread.
Gently spoon in 1-1.5 tbsp of mascarpone. It is important that the cheese is at room temperature, but don't worry too much about getting it down to the bottom of the bread; it will spread as it cooks.
Dredge the bread in the cocoa/egg mixture, allowing it to absorb for about 20 seconds per side.
Heat a frying pan over medium heat, and cook the toast for about 3 minutes, flipping and cooking for another 2-3 minutes. The bread will stiffen slightly when it is ready to flip.
Sprinkle with powdered sugar and serve with fresh raspberries and toasted hazelnuts.