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

Toast the pecans. Spread the chopped pecans on a baking sheet and toast in the preheating oven for 6–8 minutes, until fragrant. Let cool. Toasting boosts flavor and keeps them crunchy.

Brown the butter. Add the butter to a light-colored saucepan over medium heat. Melt, then keep cooking, swirling occasionally, until brown bits form and it smells nutty, 5–7 minutes. Remove from heat. Scrape every bit (including the browned milk solids) into a heatproof bowl. Cool 10–12 minutes so it doesn’t scramble the eggs.

Mix the sugars and butter. Whisk the brown sugar and granulated sugar into the slightly cooled brown butter until glossy and smooth.

Add eggs and vanilla. Whisk in the eggs, one at a time, until fully combined and shiny. Add the vanilla. The batter should thicken slightly.

Combine dry ingredients. In a separate bowl, whisk the flour, baking powder, baking soda, salt, and cinnamon (if using).

Bring it together. Switch to a spatula and fold the dry ingredients into the wet mixture just until no dry spots remain. Do not overmix.

Fold in the mix-ins. Gently stir in the toasted pecans and toffee bits. Save a small handful to sprinkle on top for looks, if you like.

Pan and smooth. Scrape the batter into the prepared pan. It’s thick—use an offset spatula to spread it into an even layer. Sprinkle the reserved nuts and toffee on top.

Bake. Bake for 22–28 minutes, until the edges are set and deep golden and the center is just barely set. A toothpick should come out with a few moist crumbs, not wet batter.

Finish and cool. If you like a sweet-salty finish, sprinkle a pinch of flaky sea salt over the top while warm. Cool completely in the pan on a rack, at least 1 hour, for clean slices.

Slice and serve. Use the parchment overhang to lift the slab out. Cut into 16–24 squares, depending on how generous you want to be.