Line a baking sheet with a silicone mat or parchment paper. Set aside.

Add butter, sugar, and salt to a heavy-bottom saucepan.

Cook over medium-low heat, stirring gently until butter melts and sugar is fully moistened.

Once the mixture begins bubbling steadily, reduce stirring. Attach a candy thermometer.

Continue cooking until the mixture reaches 300°F (hard crack stage). This can take 10–20 minutes.

Color should be deep golden amber.

Smell should be caramelized, not burnt

Remove from heat and carefully stir in vanilla.

Immediately pour onto prepared baking sheet and spread slightly.

Let cool completely until hard.

Break into chunks. Measure 120g for cookies and save the rest for snacking or topping desserts.

Add unsalted butter to a light-colored saucepan. Cook over medium heat, stirring frequently. The butter will melt, foam aggressively and begin to quiet down as the water cooks off. Watch closely as golden brown milk solids form at the bottom of the pan and the butter smells nutty and toasted. Once the butter turns deep amber and smells like toasted hazelnuts, remove immediately from heat.

Pour browned butter into a large mixing bowl and stir in the milk powder and optional espresso powder. Set aside for 15-20 mins to cool.

In a separate medium mixing bowl mix in all the dry ingredients will combined and set aside. flour, salt, cornstarch, baking soda, and baking powder.

Whisk the brown sugar and granulated sugaruntil glossy and cohesive.

Whisk in the egg, egg yolk, and vanilla until smooth and emulsified.

Add dry ingredients to the wet mixture and gently fold until well combined. Do not overmix!

Gently fold in the chopped chocolate and homemade toffee chunks.

Cover and refrigerate dough for at least 2 hours (overnight recommended for best flavor and structure).

When ready to bake cookies, preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

Scoop dough into 2 tbsp portions (45–55g each) and space 2½ inches apart.

Bake for 9-10 minutes until the edges are golden brown, set and the centers look lightly underbakled. I like to slightly underbake mine because the cookies will continue to firm up as they cool. Because homemade toffee releases butter as it melts, these cookies may spread slightly while baking. If needed, gently nudge the edges toward the center with a spatula while the cookies are still hot, or swirl a biscuit cutter or small bowl around each cookie to reshape into perfect circles.

Immediately sprinkle with flaky sea salt. Cool on the baking sheet for 5 minutes, then transfer to a wire rack.