put the hazelnuts in a blender and start the blender, mix for a few seconds until you get a beautiful composition. Add the flour, salt, sugar over the ground hazelnuts and start the blender again to mix everything well. Add the pieces of cold butter and start the blender again. At the end, pour the water and mix again for a few seconds until the composition starts to bind like a dough.
Remove the dough from the blender, and put it in a food foil, flatten the dough, wrap it in foil and put it in the refrigerator for 2 hours. After the 2 hours of standing in the refrigerator, heat the oven to 180 degrees Celsius, remove the dough from the refrigerator, and with the help of the rolling pin, roll out a 1 cm thick sheet that you place in the shape of a cake. Prick the dough from place to place with a fork and shape it in the hot oven for 10-15 minutes until the dough is baked. At first, it will seem like a fluffy dough, but after it cools, it will become hard and firm like a giant biscuit. Leave the shaped top to cool completely, after which you can remove it.
Prepare the strawberry jelly: hydrate the gelatin in 3 tablespoons of cold water, set aside. In a saucepan, put the chopped strawberries together with the sugar, lemon juice and water. Let it boil for 10 minutes until the strawberries soften and form a sauce. Puree the strawberries cooked in this way with a blender until you get a fine puree. While the composition is still warm, add the hydrated gelatin and mix to dissolve completely. At the end, add the vanilla sugar. Let it cool down at room temperature, taking care not to let the jelly harden. If you happen to leave the jelly too long at room temperature and it seizes and congeals, put the jelly back on the fire for a few seconds until it returns to a liquid state.

For the panna cotta: in a bowl, put the gelatin together with 2 tablespoons of water and let it hydrate for 10 minutes. Put the liquid cream, milk and sugar in a saucepan and heat the mixture, being careful not to bring it to the boiling point. Add the hydrated gelatin, mix well to dissolve the gelatin, pour in the rose water and let it cool down at room temperature, then refrigerate for 10 minutes. Careful! check the panna cotta very often while it is in the fridge! You need a still liquid composition, not hard and firm! When the panna cotta has cooled down and has a thick consistency like cream, take it out of the fridge and you can fit it into the cake. You can find the way to assemble the cake below.
If it still happens to you and the panna cotta hardens in the pan while it is in the refrigerator, put it back on the heat for a few seconds until it reaches a liquid state again. Chop the strawberries into cubes and leave them aside.

For the fluffy top: Preheat the oven to 180 degrees Celsius, mix the egg together with the sugar until it turns white and doubles in volume. Add the oil, continue to mix, then add the milk and mix again. Finally, in a small bowl, mix the flour together with the baking powder and with the help of a spatula, incorporate it into the egg mixture. In a 20 cm diameter form, place baking paper, grease the paper with oil and pour the composition. Put the top in the hot oven for 10-15 minutes until the top is baked, insert a toothpick in the middle of the top. If the toothpick comes out dry, then the top is baked.