Northern Norwegians celebrate the day the sun returns after polar night with solboller, doughnuts filled with raspberry jam or vanilla cream and sprinkled with sugar.
Heat the butter and milk in a saucepan until the butter melts. Pour the mixture into a large mixing bowl and let it cool to lukewarm.
Add the yeast, cardamom and eggs and mix until smooth. Then add the flour, baking powder and vanilla and knead the dough on a floured surface for about five minutes. You might need to add a bit more flour if the dough is too sticky to knead (but don't add more than necessary).
Cover and let the dough rise until doubled in size (about 45 minutes, though time can vary).
Roll the dough out into a thick (3/4 inch, or 2cm) sheet and use a glass or form to cut out round slices. Cover and let them rise again for 15 minutes before frying.
Heat the lard or oil in a large pot or pan to 330°F (165°C). I use a cooking thermometer to monitor the temperature, but if you don’t have a thermometer, you can check that the fat is hot enough by sticking the end of a wooden spoon in the fat – if it bubbles the fat is hot enough. Then reduce the heat to low or medium so you can maintain a stable temperature.
Gently drop the dough in the oil and fry on each side until golden brown (about 90 seconds per side). Transfer to a wire rack covered in paper towels. If using granulated sugar, roll the doughnuts in sugar while they're still warm.
Make a small hole in each doughnut with a chopstick or something similar and use the chopstick to make a hollow in the middle of the doughnut (see video below).
Use a piping bag or squeeze bottle to fill each doughnut with jam or vanilla custard.