Preheat your oven to 350°F (175°C) and line a 9x9-inch square baking pan with parchment paper.

Combine melted butter, all-purpose flour, and salt in a mixing bowl and blend until crumbly.

Press the crust mixture into the bottom of the prepared pan evenly.

Bake for 15-20 minutes until light golden brown; let it cool to room temperature.

Whisk together the eggs, brown sugar, and white vinegar in a separate bowl until smooth and slightly frothy.

Pour the filling over the cooled crust and spread evenly with a spatula.

Bake for an additional 25-30 minutes until filling is set around the edges but slightly gooey in the center.

Cool for about an hour in the pan, then refrigerate for at least 2 hours to fully set.

Lift out the squares using parchment overhang, cut into even squares and serve at room temperature or chilled.