Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving overhang on the long sides for easy lifting. Lightly grease the parchment.

Mix the base. In a large bowl, whisk the melted butter and brown sugar until smooth. Whisk in the egg and vanilla until glossy.

Combine dry ingredients. In a separate bowl, stir together the flour, baking powder, and salt. Add to the wet mixture and fold just until no streaks of flour remain.

Press and par-bake. Press the dough evenly into the prepared pan. Bake for 12–14 minutes, until the surface looks set and lightly puffed but not browned.

Make the toffee. While the base bakes, combine butter, sugar, corn syrup, water, and salt in a medium heavy-bottomed saucepan. Cook over medium heat, stirring gently until the sugar dissolves and the mixture starts to simmer. Then stop stirring and cook until the color turns deep golden, 6–8 minutes. Off the heat, carefully stir in vanilla.

Layer and bake. Pour the hot toffee evenly over the warm base and tilt the pan to spread. Return to the oven and bake 8–10 minutes, until the topping is bubbling across the surface and the edges are a shade darker.

Add chocolate and nuts (optional). Remove the pan. Immediately scatter chocolate chips over the hot toffee. Let them soften for 2 minutes, then spread gently into a thin layer. Sprinkle nuts and a pinch of flaky salt.

Cool completely. Let the bars cool in the pan on a rack until the toffee sets and the pan is no longer warm, about 1–2 hours. For the cleanest cuts, chill for 20–30 minutes.

Slice and serve. Lift the slab using the parchment and transfer to a cutting board. Use a sharp knife to cut into bars or diamonds. Wipe the blade between cuts for neat edges.