In a bowl of a stand mixer, combine the yeast and milk. Let sit for 5 minutes or until the yeast becomes slightly foamy. Add the melted butter and egg.
In a bowl, combine the cardamom, salt, sugar, and both flours. Add the flour mixture to the stand mixer and mix until all the ingredients are well mixed together. Increase the speed on the stand mixer to medium and knead for 5 minutes.
Remove from the bowl and form into a round ball. In a greased bowl, place the dough and cover. Allow it to rest for at least one hour or until it doubles in size.
Preheat an oven to 350°F. Divide the dough into 15-18 portions (about 75 grams each) and shape them into round ball. Cover with plastic wrap and let rest for 30 minutes.
In a small bowl, whisk an egg, heavy cream, and water. Remove the plastic wrap and place the rolls on a baking sheet with parchment paper.
Brush each of the buns with the egg wash.
Place in the oven and bake for 18 to 22 minutes or until golden brown. Place on a wire rack and allow to cool slightly.
Once cool enough to handle, cut a small portion of the top of the bun with a serrated knife. Remove the center of the bun and place in a bowl. Repeat until you have removed the centers of all the buns.
In the bowl, combine the bun fillings and the marzipan. Eveningly spread the mixture between each of the buns.
Make the whipped cream by combining all the ingredients in a stand mixer and blending until it forms stiff peaks.
When you are ready to serve, top each of the buns (minus the cap) with a portion of the whipped cream. Top with the cap and a dusting of powdered sugar.
