Dissolve the brewer’s yeast in the room-temperature milk.
Prepare the stand mixer with the dough hooks.
Pour the mixture into the bowl of the stand mixer and add the sugar, egg and lemon zest.
Add the flour in slowly while mixing, to create a dough.
After all the flour has been added, keep mixing the dough for approximately 10 minutes, or until the dough is smooth.
Now add in chunks of the softened butter one at a time, until it is all absorbed and the dough is smooth and elastic.
Cover the dough with plastic wrap and set aside to rise for approximately 2 hours. It should double in size.
After two hours have passed, form the dough into a ball (it will slightly deflate), place it in a ceramic bowl and cover it with plastic wrap and put in the fridge for at least 4 hours, but even overnight is great.
Now divide the dough into 12 even balls, rolling them on a hard surface with a small amount of flour until smooth and round.
Place them on a flat surface and let them rise for another 4 hours.
Pour a generous amount of peanut oil into a deep pot, it will need to be full enough to easily cover the top of the krapfen when frying. Heat until very hot.
Fry 2 to 3 krapfen at a time in the very hot oil.
When golden brown, remove from the oil and place on a plate lined with a paper towel to absorb any excess oil.
When the doughnuts are slightly cooled, you can gently perforate the side of the doughnut with a small knife to create a hole.
Fill a pastry bag with the filling of your choice and squeeze the filling into the hole, leaving a little dollop on the outside to show the filling.
Sprinkle each krapfen with powdered sugar and serve while still warm.
Instructions for the pastry cream
In a large saucepan (but not over heat for now) add the egg yolks and sugar and mix well with a wooden spoon.
Now add in the flour slowly, stirring continuously until it is all absorbed.
In another pot, heat the milk until it is very hot and add a dash of lemon or lemon zest for flavor, as desired.
When the milk is hot turn off the heat and pour it slowly into the saucepan with the egg yolk mixture, stirring the whole time.
Now turn the flame on low under the saucepan.
Bring it to a simmer and let cook for 3 or 4 minutes, still stirring, until several bubbles burst to the surface, the temperature should be around 82-84°C or (180-184°F).
Pour the pastry cream into a mixing bowl and let cool, stirring every so often so it does not form a film on top.