In a medium saucepan, melt the unsalted butter over medium heat, stirring frequently until it turns a deep golden brown and emits a nutty aroma, about 5-7 minutes. Transfer to a bowl, stir in the espresso powder, and refrigerate until slightly solidified, about 1-2 hours.

Once the browned butter has cooled, place it in a stand mixer bowl with the brown sugar and granulated sugar. Beat on medium speed until fluffy and light in color, about 3-4 minutes.

Add the egg and vanilla extract to the mixer. Beat on low speed until smooth, scraping down the sides as necessary.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add to the wet mixture, mixing on low speed until just combined.

Fold in the toffee bits using a spatula, ensuring they are evenly distributed.

Scoop the dough into balls (approximately ¼ cup each) and place on a baking sheet. Cover with plastic wrap and refrigerate for at least 2 hours, preferably 24 hours.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Space the chilled dough balls on the baking sheet, sprinkle with flaked sea salt, and bake for 11-13 minutes, or until the edges are golden brown.

Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.