Preheat oven to 350°F. Line a 9x13 baking pan with parchment paper and grease lightly.
Prepare the crust. In a medium sized bowl mix flour, granulated sugar and cubed butter with your hands. Use your fingers to gently incorporate the butter into the dry ingredients until the mixture is moist throughout and forms a dough when you squeeze it.
Press the crust mixture into the baking pan. Bake for 10-15 minutes or until the crust starts to become golden.
Meanwhile, prepare the filling. In a large bowl, combine melted butter and brown sugar. Stir in the eggs one at a time. Sprinkle in the flour and add vinegar and vanilla extract. Stir to combine.
Pour the filling over the crust and bake for another 30 minutes or until the filling is just about set. The middle of the filling should still be a little jiggly when you remove it from the oven - it will finish cooking on the counter!
Cool the bars to room temperature in the pan and place them in the fridge for at least 2 hours. Once set, remove the bars from the pan by pulling gently on the parchment paper and cut them into squares.