During Hanukkah in some Israeli communities, vendors sell these buns by the baskeful.
Stir together 1-1/4 cups of the flour, the yeast, and cinnamon; set aside. In a medium saucepan heat and stir 1 cup water, the granulated sugar, cooking oil, and salt just until warm (120 degree F to 130 degree F). Add oil mixture to flour mixture; add egg and vanilla. Beat with an electric mixer on low to medium speed for 30 seconds, scraping the sides of the bowl constantly. Beat on high speed for 3 minutes. Using a wooden spoon, stir in as much of the remaining flour as you can.
Turn dough out onto a lightly floured surface. Knead in enough of the remaining flour to make a moderately soft dough that is smooth and elastic (3 to 5 minutes total). Shape dough into a ball. Place in a lightly greased bowl, turning once to grease the surface of the dough. Cover and let rise in a warm place until double in size (about 1 hour).
Punch dough down. Turn dough out onto a lightly floured surface. Divide in half. Cover and let rest 10 minutes. Roll dough, 1 portion at a time, to 1/4-inch thickness. Cut dough with a floured 2-1/2-inch biscuit cutter, dipping cutter into flour between cuts. Place about 1/2 teaspoon preserves or chocolate-hazelnut spread onto the centers of half of the circles. Lightly moisten edges of circles; top with remaining circles. Press edges together to seal. Repeat with remaining dough and filling. Reroll and cut trimmings.
Fry filled doughnuts, 2 or 3 at a time, in deep hot oil (365 degree F) about 1 minute on each side or until golden, turning once with a slotted spoon. Remove from oil; drain on paper towels. Sprinkle with powdered sugar. Cool on wire racks. Makes 24 servings.