To make the raspberry filling: add all your ingredients (raspberries, sugar, salt, lemon juice, lemon zest) to a small saucepan and place on the stove over medium heat.
Stir until the raspberries have thawed and the sugar has dissolved, then allow to simmer and thicken for about 15 minutes, stirring occasionally so nothing burns.
Once you can see the bottom of the pan when you scrape a spoon or spatula through the jam, and it has darkened in color, it should be thick enough to set once cooled.
Transfer to a bowl or tupperware and let it cool briefly at room temperature before transferring to the fridge to fully cool.
For the bun dough: if using active dry or fresh yeast – heat up the milk, either in the microwave or in a saucepan, until it is “fingervarm” (finger warm), meaning it should feel warm to the touch but not hot. Add the yeast and stir to dissolve. Let it rest 5-10 minutes until it begins to froth and bubble a little bit. If using instant yeast – just add the yeast and milk in the next step.
Add the egg, egg yolk, sugar, vanilla extract, cardamom, and salt to a large bowl. Mix well. If you're using instant yeast, add the yeast and milk in this step.
If you’re using active dry yeast, once the yeast has bubbled up, add the milk and yeast mixture to the egg mixture and whisk to combine.
Add the flour and the softened butter, stirring with a wooden spoon. Once the dough starts to come together, you can start kneading.
Knead until smooth and elastic, probably around 10-15 minutes. Add a bit of flour if the dough sticks to your hands or the surface. If you prefer not to get your hands messy, feel free to use a stand mixer for this step. The texture of your dough before its first rise should be soft and smooth, not sticky and not too dry.
Cover and let your dough rise for about 45 minutes, or until doubled in size.
Once the dough has risen, dump it out on a flat surface. Divide into 12 equal pieces. If you want very even-looking buns, weigh the total lump of dough, then divide this by 12 and weigh each piece to ensure that all buns weigh the same.
Shape the pieces into buns. How to: take a piece, pinch together into a ball shape, then place it seam-side down on an unfloured surface. Cup it in your hand with the top of the roll touching your palm. Move your cupped hand in quick, tight circles – this will cause the seam to come together and create tension on the surface of the dough.
Place the shaped buns on a baking sheet lined with parchment paper. Let them rise for about 45 minutes or until doubled in size. Meanwhile, preheat your oven to 350 F (175 C).
Once the buns have risen, egg wash the buns and bake for 12-14 minutes, or until golden brown and hollow-sounding when you tap them. Let them cool completely.
While the buns are cooling, prepare the whipped cream. Whip the heavy cream using a stand mixer, a handheld mixer, or a whisk, and add sugar and vanilla extract. Whip until it reaches stiff peaks.
To fill the fastelavnsboller, cut off the top third of the bun and remove some of the center (about 1 tbsp's worth).
Fill the center of the bun with about 1 tbsp of raspberry jam.
Fill a piping bag with a star nozzle with the whipped cream mixture, then pipe whipped cream in a thick layer on the top of the jam. We usually pipe a circle around the edge, then fill in the middle to create an even layer. Add back the top of the bun, sprinkle with powdered sugar, and enjoy!