Cook - Add butter, brown sugar and salt to a small saucepan. Set over medium heat, and whisking often, cook until the thermometer reads 300°F (149°C). About 8-10 minutes. If at any point the mixture separates, take the pan off the heat and whisk vigorously until it comes back together.

Cool & Break - Immediately pour the toffee mixture onto a small baking sheet lined with parchment paper. Quickly spread it as thin as you can and allow to cool for 15 minutes. Using the back of a large spoon, rolling pin or mallet, break into small pieces. Set aside.

Prep Oven & Baking Pan - Place a rack in the center of the oven and preheat it to 350ºF (176ºC). Then prepare an 8x8 inch (20x20cm) square pan by buttering and lining with parchment paper, leaving an overhang for easy removal. Set aside.

Combine Butter & Brown Sugar - Melt butter in the microwave or a small saucepan. Transfer melted butter into a large mixing bowl along with the brown sugar and whisk until well combined. Let cool slightly for 5 minutes.

Combine Dry Ingredients - Meanwhile, in a small mixing bowl, combine all-purpose flour, fine espresso powder and fine salt. Set aside.

Prep Chocolate - If you're using a chocolate bar, roughly chop it and set aside.

Combine Wet Ingredients - Once the butter/sugar mixture has cooled slightly (it should be lukewarm and not too hot), add 1 whole egg, 1 yolk and vanilla and whisk until emulsified.

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

Fold in Chocolate & Toffee - Add chopped chocolate (or chocolate chunks or chips) and toffee bits and gently fold into the batter. Reserve some chocolate pieces and toffee to add on the top.

Transfer & Bake - Transfer the blondie batter to your prepared pan, then sprinkle the reserved chocolate and toffee pieces on top. Bake for 20 to 24 minutes. It's ready when the edges are golden and the middle is slightly puffy and shiny. A toothpick inserted into the center should come out with a few moist crumbs. Do not overbake!

Cool & Serve - Set on a wire rack and allow to cool completely in the pan, before cutting into small pieces. It should take about 2 hours. Top with flaky sea salt, cut into 16 equal pieces or 1-inch bites (2.5cm) and enjoy!!