1. To make Crust: Preheat oven to 350°F. Line 8-inch square baking pan with parchment paper so that paper extends beyond rim of pan. Stir together butter, brown sugar, lemon zest, and salt in medium bowl. Stir in flour. Pat mixture into bottom of pan. Bake 25 to 30 minutes, or until bottom is golden brown.
2. To make Filling: Whisk together sugar, lemon juice, honey, vanilla, salt, and eggs in medium bowl. Whisk in flour, and then pour over hot Crust.
3. Return pan to oven, and reduce heat to 300°F. Bake 15 to 20 minutes, or until Filling barely jiggles in center when pan is shaken. Remove pan from oven, and set on wire rack until completely cool. Lift square out using parchment paper, and slice into bars.