Heat the water and the butter in a saucepan over medium heat. Bring to a boil and then turn off the stove. Add the flour and salt, and stir until it all comes together in a ball. Let it cool to the touch. Stir in the egg and continue to stir until the dough has absorbed all the egg and is completely homogenous
Line a baking sheet with parchment paper, divide the dough in four (for smaller bollen) or in two (for large ones) and place it on top of the parchment. Bake in a 200C (425F) degree oven for about 20-25 minutes or until puffy and golden. Cool on a rack.
In the meantime, beat the whipping cream and the sugar until it is stiff. Fill a pastry bag with a small tip and poke through the bottom of the boul. Fill with whipped cream. Heat the chocolate bar and the tablespoon of water in the microwave (30 seconds on medium), stir until the chocolate has melted and the sauce has come together. Cool for about five minutes, then carefully take the cream-filled Bossche bol and dip, head first, into the chocolate. If you don’t want to get your hands dirty, just set the bollen on a rack and slowly pour the chocolate over the top, one spoonful at a time.
Cool in the fridge for about 20 minutes or until the chocolate is solid and everything has had a chance to firm up a bit. Enjoy with a nice cup of coffee and some good company.