Optional garnishes: sprinkles, strawberries, m&ms etc.
Begin by building four cone structures: take a square of aluminum foil (roughly 12x12), pinch the center, and pull upwards. Bunch the hanging foil to create the shape of a cone roughly 6 inches high and 2 inches at the large end. It can be fairly loose as long as it stands upright. Use a second square of foil to cover and create a smoother exterior.
Preheat oven to 350°F.
In a bowl, mix together the flour, sugar, salt and baking powder. Add in the butter and then gradually pour in the water, a little at a time, until a dough has formed. You may not need all the water. At this point you should have a smooth but not too sticky dough.
Divide dough into 4 equal portions and roll into a snake about 3/4-inch thick. Wrap the snake in a spiral motion over the cone, pressing it at the ends. Start at the tip and finish at the wide end.
Make an egg wash by whisking an egg with 1 tablespoon of water. Set the cones upside down on a baking sheet and brush each with the egg wash.
Bake for 15-18 minutes until golden. Remove by gently sliding the donut up and off the cone using a hand protector. Brush with melted butter and roll in cinnamon sugar. Spread the inside of the cone with some Nutella before adding a big scoop of ice cream and the toppings of your choice.