Preheat oven to 375. Butter large rectangular dish.

In large bowl, combine egg yolks and 1/3 cup sugar; beat with handheld mixer until ribbons form-about 8 minutes. Add flour, vanilla & cream. Reduce speed to low; beat until completely blended, stopping mixer occasionally to scrape down sides of bowl.

In small bowl, using handheld whisk, beat egg whites & salt about 30 seconds. Add whites to batter; beat with mixer on low speed until incorporated-approx. 1-2 minutes.

Preheat prepared baking dish in oven 4-5 minutes. In bowl, stir together strawberries, 1/3 cup sugar & lemon zest. Remove pan from oven. Pour in berries; top with batter. Bake until clafoutis is set in middle-approx. 30/35 minutes. Serve warm with a sprinkle of powdered sugar & a dollop of whipped cream, if desired.