1. Cut the cold butter into pieces and mix it with the flour. Knead until well combined. Do this quickly so the butter does not get warm. Place the mix between two pieces of plastic wrap or baking parchment and shape it into a square with a thickness of about 1cm. This works best with a rolling pin. Cover and place in the fridge until needed.

2. Add the flour, sugar and salt for the dough to a large bowl and mix. Crumble the fresh yeast into pieces and add it together with the butter, egg, egg yolks and the cold milk to the bowl. Knead until you get a nice smooth dough. Wrap loosely in plastic wrap and place in the fridge for at least 1 hour.

3. Take the dough out of the fridge and roll it out on a floured surface into a square – it should be about double the size of the butter square. Add the butter (without the plastic wrap) on top of the dough square. The corners of the butter square should point to the sides of the dough square (45° rotation, if that helps). Fold the dough edges over the butter so it looks like an envelope. Maybe take a look at the little illustration above if you are not sure what I mean. Press the seams together so the butter is completely sealed in. Roll out to a rectangle with triple the length of the original dough square. Fold the ends towards the center on top of each other so you get a square again with three layers. Wrap this in plastic wrap and place it in the fridge for about 30 minutes. Take the dough out of the fridge and repeat the process. Roll out again (let the open side of the package face you when rolling out) and then fold it again like you did in the first round. Place the package in the fridge for another 30 minutes.

4. For the filling wash and dry the rhubarb, cut it into small pieces and add it to a small pot together with the water and sugar. Heat up and let gently cook until a bit softer, but not mushy. The rhubarb pieces should still hold together. Let it drain and set it aside. For the almond part of the filling mix the almond paste/marzipan with the butter and sugar in a small bowl until well combined. Set everything aside.

5. Line two baking sheets with baking parchment and set them aside. Roll out the dough on a floured surface to a long rectangle. The short side should be around 20cm wide/long. Cut the dough into 12 »kind of« triangles with a cut-off tip. They should basically be strips with one side wider than the other (see pictures). Place some of the almond filling on the wider side of the strips and top that with some rhubarb pieces, then roll up tight. The ends of the strips should be underneath the croissants when you place them on the baking sheet or they will unroll during baking. Also keep enough space in between because they are rising considerably. If there is filling leaking out, press the sides a bit together. Continue with the remaining strips and fillings. Cover the croissants loosely with plastic wrap and let them rest for about 10-15 minutes.

6. While the horns are resting, preheat the oven to 200°C (390°F). Whisk the egg yolk with some water and brush the croissants with it, then sprinkle them with some chopped hazelnuts. Place the baking sheets in the oven and bake the croissants for about 15-17 minutes. Take them out of the oven and let them cool down on a wire rack. The croissants are best on the day they are made but can be kept in an airtight container for the next day as well.