To make the dough: Whisk the flour and salt together in a medium bowl. Work in the butter until the mixture is crumbly., Whisk the egg, vinegar, and 2 tablespoons water together in a small bowl. Drizzle it into the flour mixture, tossing to moisten evenly., Fold the dough over on itself until it comes together, drizzling in more water as needed to moisten any dry patches., Pat the dough into a slab about 1/2" thick, wrap, and refrigerate for at least 1 hour., To make the filling: Use a mixer to beat the eggs, brown sugar, and salt until smooth. Add the maple syrup, vinegar, melted butter, and vanilla and beat until thoroughly combined., Preheat the oven to 375°F. Have on hand a lightly greased or non-stick 12-cup standard muffin pan., To assemble the tarts: Roll the dough 1/8" thick. Cut 4" circles and press them gently into the muffin tins. Fill each tart two-thirds full with the filling., Bake the tarts for 20 to 25 minutes, until the crust is golden and the filling is bubbly. Remove from the oven and cool completely before unmolding., Wrap and store in the refrigerator for up to 1 week; or freeze for longer storage, up to 2 months.