In the bowl of your electric mixer or a regular mixing bowl, beat the egg with sugar and salt until fluffy, about a minute. Whisk in a few tablespoons of flour to get a smooth batter.
Whisk in 1/3 of the milk. Add 1/3 of the flour. Repeat until you've added all. Mix well after each addition. This will give you the smoothest, lump free batter.
Lastly whisk in the cocoa powder and ground coffee. Set aside.
Add a few drops of oil to a non stick pan set over medium-high heat. Add about 1/2 cup of batter per crepe and quickly swirl the pan so that the batter evenly and thinly coats the bottom of the pan. Cook for about 30-60 seconds or until the edges are golden brown and the top of the crepe doesn't look completely raw anymore.
Fip the crepe with a spatula. Cook for another 30-60 seconds and transfer to a plate. Continue with the rest of the batter. Add 1/2 teaspoon of oil to the pan when needed.
Spread about 1 tablespoon of Nutella over each crepe. Fold in the opposite sides and roll it. Place crepes in your serving dish.
To make the sauce, combine butter and chocolate in a heatproof bowl placed over a saucepan of simmering water. Allow to mostly melt, then remove the bowl from heat and gently stir the butter into chocolate. When it's completely smooth, pour the hot chocolate sauce over the crepes and serve.