Preheat your oven to 350°F (175°C). In a mixing bowl, combine flour, melted butter, brown sugar, and salt until crumbly. Press the mixture into a greased 9x9-inch baking pan.

Bake the crust for about 15 minutes until light golden brown. Allow to cool slightly.

Whisk together eggs, brown sugar, white vinegar, and vanilla extract until smooth. Fold in optional ingredients if using.

Pour the filling over the pre-baked crust and tap the pan to eliminate air bubbles.

Bake for an additional 25-30 minutes until the edges are set but the center is jiggly. The top should be golden.

Let the squares cool at room temperature for 30 minutes, then refrigerate for 2-3 hours before cutting.

Cut into squares and serve. Wipe the knife between cuts for clean edges.