Brown the butter. Add the butter to a light-colored saucepan over medium heat. Melt, then cook, stirring, until it foams, the milk solids turn golden, and it smells nutty, 5–7 minutes. Immediately pour into a heatproof bowl, scraping in the brown bits. Cool 20–25 minutes until just warm, not hot.

Preheat and prep. Heat the oven to 350°F (175°C). Line two baking sheets with parchment.

Whisk dry ingredients. In a medium bowl, whisk flour, baking soda, and kosher salt. Set aside.

Mix sugars and butter. In a large bowl, combine the slightly cooled brown butter with dark brown sugar and granulated sugar. Whisk or beat until glossy and well combined, about 1–2 minutes.

Add eggs and vanilla. Beat in the eggs one at a time, then add vanilla. Mix until the batter looks smooth and slightly thickened, 1 minute.

Combine wet and dry. Add the dry ingredients to the bowl and mix on low or fold with a spatula until just combined. Do not overmix.

Fold in toffee. Stir in the toffee bits and any optional add-ins. The dough will be glossy and scoopable.

Chill briefly for best texture. Chill the dough 20–30 minutes. This helps prevent overspreading and deepens the flavor.

Portion. Scoop heaping tablespoon-sized balls (about 1 1/2 tablespoons each) or use a #40 scoop. Space 2 inches apart on prepared sheets.

Bake. Bake 9–12 minutes, rotating trays halfway. Edges should be set and lightly golden with soft centers. If desired, tap the pan once on the rack to settle the cookies and create rippled edges.

Finish and cool. Sprinkle lightly with flaky sea salt while warm. Let cool on the sheet 5 minutes, then transfer to a rack to cool fully. They’ll firm up as they rest.