Make a roux: melt the butter. Before the butter turns brown, add the flour in one go. Stir with a wooden spatula until you have a paste. Keep the fire low. Keep stirring, and let the flour cook for a few minutes. Then start by adding the broth, which you have weighed in advance in a measuring cup. The liquid can be cold or warm. In the beginning, add a small amount of broth. As soon as the first broth falls into the pan, the paste turns into a dough. Keep stirring, then there will be no lumps. When the stock is completely absorbed, pour another small amount of stock and stir again until smooth. As more broth is added, a larger plunger can be added. However, wait every time the broth is completely absorbed and has been puffed. Squeeze the gelatine sheets and add them to the mass. Allow to cool slightly.