Preheat the oven to 350°F (177°C) and line a 9-inch square baking pan with parchment paper. Measure all ingredients and chop the milk chocolate bar into small chunks.

Using a stand mixer and a large bowl, beat the butter and brown sugar together until the color is lightened and the mixture is fluffy. Scrape down the sides of the bowl with a rubber spatula as needed.

Add the eggs and vanilla extract and continue to beat until all is well combined.

Sift together flour, instant espresso powder, baking powder, and salt. Add the flour mixture to the wet ingredients and beat until it is incorporated.

Remove the bowl from the mixer and with a rubber spatula fold in the toffee bits and ⅔ of the chocolate chunks. Spread the blondie batter in the prepared pan and scatter the reserved chocolate chunks over the top of the blondies. Lightly press the chunks of chocolate into the top and sprinkle with flaked sea salt.

Bake the blondies for 30 minutes. The top of the blondies will feel set, but a toothpick might not come out completely clean.

Cool the blondies completely in the pan on a wire rack. Using the parchment paper, lift the blondies from the pan and place them on a cutting board. Cut them into squares or bars. If not eaten immediately, wrap individually and store in an airtight container.