Preheat the oven at 180 degrees Celsius (350 degrees Fahrenheit)
Rinse the stinging nettles thoroughly in a colander.
In a large pot, bring water to a boil and add the nettles. Boil the nettles for about 2 minutes.
Bring back the cooked nettles to the colander and pour cold water on them.
Now you will easily be able to touch the nettles without being stung. Remove the stems and keep only the nettle leaves.
Squeeze out the water from the nettle leaves and place them in a blender or food processor together with the heavy cream. Process until it becomes a purée.
In a large bowl, mix butter and sugar.
Add the shredded marzipan and the nettle purée and mix well.
Add one egg at a time and mix well in between.
Finally, add all-purpose flour, salt, and baking powder and mix until combined.
Place the dough in a baking dish covered in parchment paper.
Bake the stinging nettle cake for about 20-25 minutes.