In a large mixing bowl, use an electric mixer to cream the butter for about 30 seconds. Mix in the sugar, baking powder and salt. Beat in the eggs and lemon zest. Beat in the flour until incorporated. Divide the dough in half. Wrap each half in plastic wrap and chill for about 1 hour (or until the dough is easy to handle).
Roll half the dough at a time between sheets of waxed paper into a 10 inch square. Remove the top sheet of waxed paper. Spread the dough with jam to within ½ inch of the edges. Don't slop it on thick or it will be a mess when you try to roll it (it'll squish out). Give it a nice, thin layer of jam. Roll up the dough using the bottom sheet of waxed paper to help lift and guide the dough. Moisten the edges, and pinch to seal. Wrap each spiral log in waxed paper or plastic wrap. Chill for 4 to 24 hours or until dough is firm enough to slice.
Preheat oven to 375 degrees F. Line a large baking sheet with parchment paper.
Using a sharp knife, quickly cut spiral logs into ¼ inch slices, repositioning the logs as needed to keep them from flattening. If the logs become too soft during cutting, place them in the freezer about 10 minutes or until they firm up. Place the slices 2 inches apart on the prepared cookie sheet.
Bake for 10 to 12 minutes or until the edges are firm and the bottoms are light brown. Cool on the baking sheet for a couple of minutes, then transfer to a rack to cool completely.
Notes
Store cookies at room temperature or refrigerated in an airtight container up to 2 days, or freeze up to 3 months.