Combine 2 cups flour, undissolved yeast, sugar, and salt in a large mixer bowl. Heat milk and butter to very warm (120°F to 130°F). Add to flour mixture with egg yolks; beat for 2 minutes at low speed. Continue adding flour until soft dough forms.
Knead on a lightly floured surface until smooth and elastic (about 4 to 6 minutes). Cover and let rest for 10 minutes. Roll out dough on a lightly floured counter into a 12-inch circle, about 1/2-inch thick. Using a 3-inch cookie or biscuit cutter, cut out as many rounds as possible.
Place doughnuts about 2 to 3 inches apart on lightly greased or parchment lined baking sheet. Re-roll and cut remaining dough. Cover doughnuts and let rise for 45 minutes to 1 hour.
Heat at least 2 inches of oil in a deep fryer or deep pan to 350°F. While the oil heats up, prep your filling. Place the jam in a pastry bag fitted with a metal tip with a wide opening (you want the jam to be able to get through!). Set aside.
Fry 2 to 3 doughnuts at a time, turning occasionally until well browned, about 2 to 3 minutes. Remove from oil and drain on paper towels slightly.
Toss the donuts in granulated sugar. Insert the pastry tip into the side of each donuts and firmly squeeze a few Tablespoons of jam into the center of the donut. Cool a few minutes, then transfer to wire rack. Serve warm.