Brown the Butter: In a light-colored saucepan over medium heat, melt the unsalted butter. Swirl frequently as it foams and sizzles. After 5 to 7 minutes, nutty golden-brown flecks will settle at the bottom with a fragrant toasted aroma. Pour immediately into a heatproof bowl and let cool for 20 minutes until creamy-soft.

Prep Oven & Pan: Preheat oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch metal square baking pan with parchment paper, leaving an overhang on the sides for a lifting sling.

Mix Shortbread Dough: In a large mixing bowl, beat the cooled browned butter with brown sugar and granulated sugar for 2 minutes until creamy. Beat in vanilla extract, cinnamon, and fine sea salt. Add the flour and fold gently with a spatula until a buttery, crumbly dough forms.

Fold Mix-ins & Press: Fold in half of the toasted pecans and half of the toffee bits. Transfer dough into the prepared baking pan and press firmly into an even, smooth layer using the flat back of a measuring cup.

Top & Bake: Scatter the remaining toasted pecans and toffee bits evenly over the surface, pressing them gently into the dough to adhere. Bake for 22 to 25 minutes until the edges are deep golden brown and the center is set.

Cool, Slice & Finish: Sprinkle immediately with flaky Maldon sea salt while warm. Cool in the pan on a wire rack for 30 minutes, lift using the parchment sling, and slice into 16 clean rectangles while slightly warm. Cool completely before serving.