Adjust oven rack to middle position and heat oven to 325 degrees. Bring milk and cream to simmer in medium saucepan over medium-high heat. Remove saucepan from heat, whisk in lemon zest, cover saucepan, and let stand for 15 minutes. Meanwhile, fold dish towel in half and place in bottom of 13 by 9-inch baking pan. Place three 6-ounce ramekins on top of towel and set aside pan.

Whisk 6 tablespoons sugar, flour, baking powder, and salt in medium bowl until combined. Strain milk mixture through fine-mesh strainer into bowl with sugar mixture, pressing on lemon zest to extract liquid; discard lemon zest. Add egg yolk, vanilla, and lemon juice and whisk until combined. (Batter will have consistency of milk.)

Using hand mixer, whip egg whites on medium-low speed until foamy, about 1 minute. Increase speed to medium-high and whip whites to soft, billowy mounds, about 1 minute. Gradually add remaining 2 tablespoons sugar and whip until glossy, soft peaks form, about 1 minute.

Whisk one-quarter of whites into batter to lighten. Gently whisk in remaining whites until no clumps or streaks remain. Ladle batter into ramekins (ramekins should be nearly full). Pour enough cold water into pan to come one-third of way up sides of ramekins. Bake until cake is set and pale golden brown and pudding layer registers 172 to 175 degrees at center, 35 to 40 minutes.

Remove pan from oven and let ramekins stand in water bath for 10 minutes. Transfer ramekins to wire rack and let cool completely. Sprinkle with confectioners' sugar, if using, and serve.