To make the toffee bits, add butter, brown sugar, and salt to a small saucepan. Set over medium heat and whisk often until the thermometer reads 300°F (149°C), about 8-10 minutes. If the mixture separates, remove from heat and whisk vigorously until combined.

Immediately pour the toffee mixture onto a lined baking sheet, spreading it thinly. Allow to cool for 15 minutes, then break into small pieces. Set aside.

Preheat the oven to 350ºF (176ºC) and prepare an 8x8 inch square pan by buttering and lining with parchment paper.

Melt the unsalted butter and transfer it to a large mixing bowl. Add the brown sugar and whisk until combined. Let cool slightly for 5 minutes.

In a small mixing bowl, combine the all-purpose flour, fine espresso powder, and fine salt. Set aside.

If using a chocolate bar, roughly chop it and set aside.

Once the butter/sugar mixture has cooled slightly, add the whole egg, yolk, and vanilla, and whisk until emulsified.

Sift in the dry ingredients and mix until there are very few flour streaks left. Do not overmix.

Fold in the chopped chocolate and toffee bits, reserving some chocolate pieces and toffee for the top.

Transfer the blondie batter to the prepared pan and sprinkle the reserved chocolate and toffee pieces on top.

Bake for 20 to 24 minutes, until the edges are golden and the middle is slightly puffy and shiny. A toothpick inserted should come out with a few moist crumbs. Do not overbake!

Allow to cool completely in the pan on a wire rack for about 2 hours. Top with flaky sea salt, cut into 16 equal pieces, and serve.