In a medium skillet, melt the butter under medium-low heat, stirring occasionally with a spatula. Stirring often, once the melted butter transforms to a caramel color, you've made brown butter. Be careful not to burn the milk solids. Once you start smelling a nutty flavor, you're pretty much done.
Pour the brown butter into an mixing bowl, and refrigerate for 10-15 minutes to take the chill off. During this time is when I assemble the dry ingredients, chop the toffee chocolates, and remove the eggs from the fridge.
In a smaller bowl, mix the flour, baking soda, and salt.
Chop the toffee chocolates with a sharp knife. I tytpically get English Toffee from Trader Joe's, but you could also use any other brands. Also, plain chocolate chips works too.
Remove the chilled brown butter from the refrigerator. Add the brown and granulated sugar, and mix very well for a minute.
Add one egg at a time, and mix very well for another minute. Doing this will thicken the batter before adding the flour. Add vanilla extract and espresso powder (optional), and mix well.
Add the dry ingredients to the brown butter batter and use a spatula to mix.
Add the toffee chips (or chocolate chips), and mix until all incorporated.
Using a cookie scoop, roll out the dough balls on a nonstick sheet pan. Bake for 8 minutes, then remove and tap the sheet pan unto a counter to flatten the cookies a bit. Return to the oven for an additional 2 minutes. Remove promptly after the 2 minutes.
Sprinkle flakey salt on each cookie. Because toffee tends to harden quickly, use a spatula and transfer the cookies to a cooling rack.
Let it cool for 10-15 minutes before scoffing 'em down! Enjoy!