Preheat your oven to 350°F (175°C). In a mixing bowl, combine melted butter, flour, and a pinch of salt. Use a fork or your fingers to mix until crumbly. Press the mixture into the bottom of a greased 9x9-inch baking pan. Bake for 15 minutes until lightly golden.

In a separate bowl, whisk together the brown sugar, eggs, melted butter, and vinegar until smooth and creamy. If adding mix-ins, fold them into the filling now.

Once the crust has cooled, pour the filling over it, spreading with a spatula without disturbing the crust.

Return the pan to the oven and bake for an additional 25-30 minutes until the filling is just set with a slight jiggle in the center.

Allow to cool in the pan at room temperature for about 30 minutes, then transfer to the refrigerator for at least 2 hours for setting.