Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper for easy removal. In a mixing bowl, combine softened unsalted butter, all-purpose flour, and a pinch of salt. Use your hands to mix until the mixture resembles coarse crumbs. Then, firmly press this mixture evenly into the bottom of the prepared pan and bake for 12–15 minutes until it’s lightly golden around the edges.

While the crust is baking, melt unsalted butter in a small saucepan or microwave until smooth. In a separate bowl, whisk together the melted butter and brown sugar until well combined and glossy. Add in the large eggs, vanilla extract, baking powder, and a pinch of salt, mixing until everything is nicely incorporated. If you’re using optional add-ins like nuts or raisins, gently fold them into the filling at this point.

Once the crust is done baking, carefully pour the luscious filling over the hot crust, using a spatula to spread it out evenly. Return the pan to the oven and bake for 20–25 minutes. The filling should be set around the edges but still have a slight jiggle in the center when you gently shake the pan.

After baking, remove the pan from the oven and let it cool at room temperature. For the best slicing results, allow the squares to cool completely in the pan for about an hour. If you prefer cleaner cuts, pop the whole pan into the refrigerator for an additional 30 minutes.