Start by browning the butter. You'll take ½ cup of butter (one stick) and melt in a medium saucepan, and then continue to cook past the point of boiling. You will do this over medium heat, past the point of the butter foaming, until it turns a nice golden brown, and has a nutty smell. Make sure you are stirring and watching it, because it will turn from perfectly brown to burnt fairly quickly. Once finished browning, pour into a glass bowl, and add espresso powder. Mix until no clumps of powder are left. Place the bowl in the fridge to solidify again. You will want it to become a soft solid again to start the cookie dough. This should take a few hours.
Place butter and sugars in the bowl of a stand mixer, fitted with a paddle attachment. Beat on medium-high speed until light and fluffy, 3 to 4 minutes.
Add the egg, mix until smooth on low speed, about 30 seconds. Beat in the vanilla.
And baking powder, baking soda, salt and flour mixture to bowl and mix on low, being careful not to overmix. I usually like to stop mixing when I still have flour remnants around the bowl. Fold in toffee chips.
Scoop the dough into large balls. I like to use a ¼ cup cookie scoop. At this point I usually like to chill my dough for a 24 hours if time permits, but at least a few hours.
Preheat oven to 350 F. Line a baking sheets with parchment paper.
Place 8 cookie balls on your baking sheet, and sprinkle with some flaked sea salt. I sprinkled a few more toffee bits on top too. Bake the cookies for 11-13 minutes. You're looking for a nice golden brown outside rim.
Allow cookies to cool on the pan before moving to a cooling rack to hold their shape.