For the custard cups: Trim ends off of 6 medium lemons so that they sit upright. Run a sharp paring knife between pulp and rind of each lemon, being careful not to break through bottoms. Hollow lemons out with a spoon, removing most of the pulp, to make lemon cups. (Reserve pulp for another use.) Transfer lemon cups to a plate, cover with plastic wrap, and refrigerate.
For the lemon custard: Put cream, milk, and zest into a small pot and bring to a boil over medium-high heat. Meanwhile, put yogurt, sugar, and yolks into a medium bowl and whisk until smooth. Gradually pour cream mixture into egg mixture, whisking constantly. Return mixture to pot and cook over medium heat, stirring constantly, for 10 minutes. Reduce heat to medium-low and cook, stirring, until thickened, 5-6 minutes more. Transfer mixture to a medium bowl; set in a larger bowl of ice water to let chill. Press a piece of plastic wrap directly on surface of custard; refrigerate.
For the lemon slices: Preheat oven to 175°. Place lemon slices on a parchment paper-lined baking sheet, sift confectioners' sugar over both sides of slices, and arrange in a single layer. Bake, without turning, until dried, about 3 hours. Set aside to let cool.
For the lemon fritters: Pour oil into a small heavy pot to a depth of 1" and heat over medium-high heat until a deep-fry thermometer inserted in oil registers 325°. Meanwhile, put 1⁄4 cup of the flour into a wide shallow dish; set aside. Put remaining flour, beer, granulated sugar, and 5 tbsp. water into a medium bowl; whisk to make a smooth batter. In a separate medium bowl, using a whisk, beat egg white until stiff peaks form. Gently fold egg white into batter, being careful not to overmix. Working in batches, lightly dredge lemon wedges in flour, coat in batter, shaking off any excess, then fry until golden, turning once, about 2 minutes. Transfer to paper towels to let drain; sprinkle with superfine sugar while still hot.
To serve, fill lemon cups with lemon custard and top with a fritter. Garnish each cup with a lemon leaf and 2 dried lemon slices. Serve immediately.