1-First, make the toffee bits: Cook ¼ cup (56g) unsalted butter, ½ cup (100g) light or dark brown sugar, and ¼ teaspoon fine salt over medium heat. Whisk until the mixture reaches 300°F (149°C) or the hard-crack stage, which takes 8-10 minutes.

2-Pour the hot toffee mixture onto parchment paper, spread it thin, and let it cool for 15 minutes before breaking it into small pieces.

3-Preheat your oven to 350°F (176°C) and prepare an 8×8-inch pan by greasing it with butter and lining it with parchment paper.

4-In a bowl, mix the melted ½ cup (113g) unsalted butter with 1 cup (200g) packed light or dark brown sugar and let it cool for 5 minutes.

5-In another bowl, combine 1 ¼ cup (150g) all-purpose flour, 1 ½ teaspoon fine espresso powder, and ½ teaspoon fine salt.

6-Add 1 whole large egg, 1 large yolk, and 1 teaspoon vanilla paste or extract to the butter mixture, whisking until emulsified.

7-Sift the dry ingredients into the wet mixture and stir until just combined. Fold in 4 ounces (113g) roughly chopped semi-sweet or dark chocolate bar and ½ cup (78g) toffee bits, reserving some for topping.

8-Pour the batter into the prepared pan, sprinkle the reserved chocolate and toffee bits on top, and bake for 20-24 minutes until the edges are golden and the center is shiny and slightly puffy.

9-Cool completely in the pan for about 2 hours before slicing, then sprinkle with flaky sea salt before serving.