- Preheat oven to 400 degrees. Place a rimmed baking sheet on rack in lower third of the oven. Butter six or seven ¾-cup (6 ounce) soufflé ramekins, sprinkle evenly with sugar, tilting to coat sides evenly.
- In a bowl, stir together yolks, flour, lemon rind, and ¼ cup of sugar.
- In a small heavy saucepan bring milk to a boil. Gradually pour milk into yolk mixture, whisking constantly. Return mixture to pan and whisk over medium-low heat to thicken, 2 to 3 minutes.
- Remove from heat and whisk in butter and lemon juice. Tip: Base can be made about 3 hours ahead to this point.
- Using electric mixer, beat egg whites and cream of tartar until soft peaks form.
- Gradually beat in remaining ¼ cup sugar, beat until stiff but not dry. Mix ¼ of whites into lukewarm lemon mixture. Gently fold in remaining whites.
- Spoon mixture into prepared dishes, filling to the top. Sprinkle with almonds. Place cups on preheated baking sheet in oven. Immediately reduce temperature to 350 degrees. Bake until puffed and golden, about 14 to 18 minutes. Dust with confectioners sugar, place on individual dessert plates, and serve immediately.