2. Put the flour in the bowl of a food processor equipped with a steel blade. Add the dissolved yeast, milk, egg, egg yolk, salt, lemon zest and the remaining 2 tablespoons of sugar. Process until blended. Add the butter and process until the dough becomes sticky yet elastic.
3. Remove the dough to a bowl, cover and let rise in a warm place for at least an hour. If you want to prepare the dough in advance, place it in the refrigerator overnight, then let it warm to room temperature before rolling and cutting.
4. Grease 2 cookie sheets. Dust a pastry board with flour. Roll the dough out to a 1/2-inch thickness. Using the top of a glass as a cutter, cut into rounds about 2-inches in diameter and roll these into balls. Place the balls about 1 to 1 1/2-inches apart on the greased cookie sheets. Cover and let rise 30 minutes more.
5. Preheat the oven to 375°F. Bake the doughnuts for 12 to 15 minutes or until they're golden. Remove from the oven and let cool.
6. Soften the jam in a food processor. Using an injector (available at cooking stores), insert a teaspoon of jam into each doughnut. Roll the soufganiyot in confectioner's or granulated sugar and serve immediately.