1. Preheat your oven to 350°F (175°C) and grease an 8×8 or 9×13 inch baking pan.
2. Melt the butter in a microwave or on the stovetop, then let it cool slightly.
3. In a mixing bowl, whisk together the melted butter and brown sugar until smooth.
4. Add eggs and vanilla extract, whisking until fully combined.
5. In a separate bowl, mix the flour, salt, and optional cornstarch or baking powder.
6. Gradually fold the dry ingredients into the wet mixture until just combined.
7. Stir in chocolate chips, nuts, or other mix-ins if desired.
8. Pour the batter into the prepared pan and spread evenly.
9. Bake for 20-30 minutes until the edges are set but the center is slightly soft.
10. Let cool completely in the pan before slicing for clean cuts.
11. Store at room temperature in an airtight container or freeze for up to 3 months.