In a small bowl mix dried yeast 1 dl of warm milk and 2 TBSP of sugar. Cover and let sit for 5-10 minutes so the foam starts to form.
Meanwhile, combine flour, sugar, salt, lemon zest and turmeric in a big stand mixer bowl.
After the yeast has started to foam add it to the flour. Add the remaining lukewarm milk and mix on medium speed for a minute.
Add oil and melted butter and mix for 10 more minutes od medium-high speed.
Cover the bowl with plastic wrap and leave to rise in a warm place for about an hour or until the dough doubles in size.
After the dough has risen roll it out on a lightly floured surface to about 2.5 cm thick (without kneading it whatsoever). Use doughnut cutters to cut out circles or use a knife to cut any desired shapes.
Leave the doughnuts to rise for about 15 minutes.
Meanwhile, heat the oil to 140°C. A good indicator the temperature is perfect is to check with a wooden spoon. If little bubbles start to form around the spoon handle the oil is ready to go.
Place doughnuts top side down in hot oil, but prior to that brush away the flour. A lot of flour will burn your oil and it will not be re-usable. Fry the doughnuts for about 3 minutes covered. When they turn golden, turn them around away from yourself, so you don't burn yourself! Fry for about 3 more minutes on the other side uncovered.
Transfer the doughnuts to a plate covered with paper towels to drain the excess oil. Transfer to a wire rack and leave to cool.
Mix corn starch with 50ml of milk to form a lump-free paste.
Bring the remaining 200ml of milk to a simmer and add the starch mixture. Whisk continuously until the pudding has thickened to a very think mixture. Cover and leave for 10 minutes to cool a bit.
Pierce the crust of each cooled doughnut with the piping tube and pipe a little pudding into each doughnut. Do not whisk or stir the pudding when transferring it to the piping bag or it will go runny.