For the cookies: Position an oven rack in the upper and lower thirds of the oven and preheat to 350 degrees F. Line 2 baking sheets with parchment paper.
Whisk together the flour, baking powder, nutmeg and salt in a medium bowl.
Beat the butter and granulated sugar together in a large bowl with an electric mixer on medium-high speed until light and fluffy, about 3 minutes. Beat in the eggs, one at a time, then beat in the sour cream, vanilla and lemon zest. Add the flour mixture to the butter mixture and use a rubber spatula to gently fold and stir until just combined.
Scoop the dough by level tablespoonfuls onto one of the prepared baking sheets and refrigerate until slightly chilled and firm, about 15 minutes.
With lightly floured hands, roll each piece out into a 51/2-inch-long rope. Join the ends together to form 2-inch rings. Place on the prepared baking sheets, leaving about a 1 inch between the cookies.
Once you've filled the 2 baking sheets (you should be able to fit about 14 on each sheet), bake the cookies until puffed and very light brown on the bottom, 10 to 12 minutes, switching the baking sheets in the oven halfway through baking. Transfer the cookies to a rack to cool. Let the baking sheets cool and bake the remaining cookies.
For the glaze: Whisk together the confectioners' sugar, lemon juice, grape jelly and 1/3 cup water in a medium bowl. Dip the rounded top of the cooled cookies into the glaze, letting any excess drip off, and place back on the rack. Immediately decorate with chocolate sprinkles before the glaze sets. Let the cookies sit for about 30 minutes for the glaze to harden. Store at room temperature in an airtight container for up to 3 days.
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