Whisk together one cup of flour, ⅓ cup of sugar, one packet of instant yeast, a pinch of salt, ⅔ cup of warm milk, 3 tablespoons of melted butter, 2 egg yolks, and ½ teaspoon of vanilla extract in a large bowl. Make sure all the ingredients are well combined. Let stand for 10 minutes, or until bubbles start to form.
Add 1 ¼ cup more flour or enough so that the dough holds together and is no longer sticky. You might need to add a bit more, but do it one tablespoon at a time. Turn the dough onto your kitchen surface (dusted with flour) and knead it for 5 minutes. Place the kneaded dough into a bowl, cover it with plastic wrap, and let it rise in a draft-free place for one hour or until doubled in size.
Turn the dough onto a floured surface and roll it to about ½-inch thick. Using a 2 ½ inch round cookie cutter, cut about 12 rounds of donuts. You don’t need to make a hole.
Place the donuts on a baking sheet lined with parchment paper, cover them with plastic wrap, and let rise for 40 minutes or until doubled in size.
Preheat the oven to 375 F and bake for 10 minutes or until slightly golden on top.
While still hot, brush with melted butter and roll into the remaining ⅓ cup of granulated sugar. Using a knife, make a deep slit in the side of the donuts, and using a pastry bag, pipe your favorite jam into the slit. Be careful not to stuff them too much, or they might burst.