Butter two cookie sheets. Sift together the flour, baking powder, and salt. Zest the two lemons into a large mixing bowl, then juice the lemons into a measuring cup. Add enough cold water to the juice to make a scant 1/2 cup liquid, if necessary, and set aside.
Add the butter and sugar to the zest in the mixing bowl, and cream until fully blended. Add the egg and lemon juice and beat well. Add the dry ingredients and mix only until they are fully incorporated.
Drop the batter by level tablespoons 2 inches apart onto the prepared cookie sheets. Bake for 10 minutes, until cookies are slightly firm but not browned.
Transfer the cookies to a rack to cool completely. Do not stack or store until cool. Store in an airtight container at room temperature.