In a small bowl, whisk together the flour, poppy seeds, and salt.
In a mixing bowl, cream the butter and sugar until light and fluffy. Beat in the egg yolks, vanilla and lemon zest; scrape down the sides of the bowl.
At low speed gradually beat in flour mixture, just until incorporated.
Cover bowl with plastic and refrigerate for at least 1 hour.
In a food processor fitted with a metal blade, process the almonds until very fine. Do not overprocess to a "butter".
Transfer nuts to a small bowl.
Preheat oven to 350°F
Measure level tablespoons of dough and roll them between the palms of your hands to form balls.
Drop each ball, as soon as it is formed, into the nuts and shake the bowl to twirl the ball around, coating it well with the nuts. Place the nut-covered balls on ungreased cookie sheet, 2 inches apart.
Bake 8-12 minutes, or until bottoms are browned. For even baking, rotate the sheets from back to front halfway through the baking period.
Transfer to wire rack to cool completely. Sprinkle lightly with powdered sugar.
Store in an airtight container at room temperature or in the freezer.