Put the butter, icing sugar, egg yolk, and vanilla seeds in a large bowl or food processor. Mix for at least 5 minutes until it is nice and soft and creamy, then put it in a piping bag. Put the raspberries, orange juice, honey, and rose water in a tall measuring cup and blend finely with an immersion blender, then strain.
Now we are going to build: dip 6 cookies in the coulis each time and make a rectangle on a flat surface: a plate, a dish, or a wooden board. Pipe lines of buttercream on top, and repeat again with 6 dipped cookies that you gently but firmly press on top. For the top layer, I used a star-shaped piping tip and piped lengthwise for a neat result, but feel free to get creative.
Let it set for at least 4 hours in the refrigerator. Cut the cake with a warm knife that you dip in boiling water each time and then wipe clean.