In a large mixing bowl, whisk together the flour, sugar, instant yeast, and salt. Create a well in the center and add warm milk, melted butter, egg yolks, and vanilla extract. Mix until well combined.

Cover the mixing bowl with a clean towel and let the dough rest for 10 minutes.

Gradually add the remaining flour one cup at a time, stirring until the dough holds together and is slightly sticky. Turn out onto a floured surface and knead for about 5 minutes until smooth and elastic.

Place the kneaded dough in a greased bowl and cover with plastic wrap. Allow it to rise for approximately 45 minutes, or until it has doubled in size.

Preheat your oven to 375°F. Line a baking sheet with parchment paper.

Roll the dough out to about 1/2 inch thick and cut out circles using a cookie cutter. Place on the prepared baking sheet and let rise for about 20 minutes.

Bake the donuts for about 10-12 minutes or until lightly golden. Allow to cool on the baking sheet for a minute.

Melt the remaining tablespoon of butter and brush the tops of the donuts. Roll each donut in granulated sugar.

Cut a deep slit into each donut and use a piping bag to fill them with jelly.