Preheat and prep the pan. Heat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving an overhang for easy lifting. Lightly butter the parchment and sides.

Make the crust. In a bowl, cream 1 cup (2 sticks) softened unsalted butter with 1/2 cup granulated sugar until smooth and light. Add 2 cups all-purpose flour and 1/2 teaspoon salt. Mix just until a soft, crumbly dough forms.

Press and par-bake. Press the dough evenly into the pan. Use the bottom of a measuring cup to compact it. Bake for 12–15 minutes, until the edges are just turning pale golden. Set aside while you make the topping.

Whisk the topping. In a large bowl, whisk 3 large eggs with 1 cup brown sugar and 1/2 cup granulated sugar until glossy and slightly thickened. Stir in 1 teaspoon vanilla extract, 1 teaspoon baking powder, and a pinch of salt. Fold in 2 1/2 cups sweetened shredded coconut. If using mix-ins, add up to 1 cup total.

Spread the filling. Pour the coconut mixture over the warm crust and spread it into an even layer, nudging into the corners.

Bake to golden and set. Return the pan to the oven and bake 22–28 minutes, until the top is golden brown and the center is set but still slightly soft. The edges should be a deeper caramel color.

Cool completely. Let the pan cool on a rack until fully set, about 1–2 hours. Use the parchment to lift the slab out, then transfer to a cutting board.

Slice and serve. Use a sharp knife to cut into bars or squares. For super clean edges, chill the bars for 30 minutes before slicing.