In a 10-inch skillet, heat the hazelnuts with ¼ cup of the sugar over medium-high, and cook, stirring, until the sugar melts and coats the nuts, about 8 minutes. Pour the nuts onto a parchment paper-lined baking sheet and let cool. Transfer the nuts to a blender and add the remaining sugar, the cocoa powder, milk powder, and oil. Blend until a coarse paste forms, scraping down the sides as needed (you should have ¾ cup).
Heat a 12-inch non-stick crêpe pan or skillet over medium-high. Add ¼ tablespoon of the butter and swirl the pan to coat the entire surface. Add 1 crêpe to the pan and quickly spread 3 tablespoons of the hazelnut paste over one half of the crepe. Fold the crepe in half to cover the filling and let cook until golden, 1 minute.
Thinly slice one half of a banana into coins, and arrange the coins in the middle third of the crêpe. Lift both folded edges of the crêpe and fold them over the bananas, forming a cone. Serve immediately, and then repeat with the remaining crepes, hazelnut paste, and bananas to make 7 more crêpes.