For the espresso nut mug cakes, first pour the milk into a cup and heat in the microwave at 800 watts for about 50 seconds. Remove and stir in the espresso powder.
Place the egg in a mixing bowl with the canola oil, sugar and salt. Whisk with a hand mixer. Then stir in the espresso milk. Sift the flour with the cocoa powder, add the ground hazelnuts and whisk briefly.
Grease two cups with butter. Divide the batter between the cups and microwave them one after the other at 800 watts for 1:50 minutes. Remove and turn out or let cool in the cup.
If preparing without microwave, heat the milk in a small saucepan and dissolve the espresso powder in it. Preheat the oven to 180 °C. Bake the espresso nut mug cakes on the middle rack for approx. 25 minutes.