1. Adjust oven rack to middle position and heat the oven to 325 degrees.
2. In a medium bowl, whisk the butter until creamy. Whisk in the honey, egg yolks, vanilla, and lemon zest. Add the flour and salt, and whisk until incorporated. Whisk in the yogurt and mix until smooth.
3. In a mixer bowl, beat the egg whites until they look creamy and form soft peaks. Take care not to overwhip them. Gently fold them into the filling and pour mixture into baked and cooled crust. Bake for 25 to 30 minutes, or until the filling is set and pale golden brown. Cool and then refrigerate for 4 hours or overnight before topping.