In a large bowl, combine 1 cup flour, ¼ cup granulated sugar, salt, egg yolks, vanilla extract and 3 tablespoons cooled melted butter. Whisk well.
Allow mixture to sit for 5-10 minutes until bubbles start to form on the surface.
Add remaining 1 ½ cups flour and mix until the batter pulls from the side of the bowl. Hand knead for 5 minutes until a smooth ball forms. Add a little flour to mixture as needed if it becomes too sticky.
Cover and let rise for 1 ½ hours until at least doubled in size. (you can test if it’s ready by gently poking the dough. If it springs back and leaves a small indentation, it’s ready to go!)
Divide the dough into 12 equal pieces. Roll each piece into a ball and place on a parchment lined baking sheet. Cover lightly and let proof for an additional 45 minutes.
Uncover and bake in a preheated 375°F oven for 10-12 minutes or until cooked all the way through.
Remove from oven and while hot, brush each donut with melted butter and toss in granulated sugar. Allow to cool completely.
Using a sharp knife, cut a deep slit into the side of each donut and fill using a piping bag filled with jelly.