Preheat the oven to 375°F. Line two baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the room-temperature butter, granulated sugar, and brown sugar on medium speed for 2–3 minutes, until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Mix in the vanilla extract. Scrape down the sides and bottom of the bowl as needed.
Add the flour, baking soda, baking powder, kosher salt, and espresso powder, if using. Mix on low speed just until the flour is incorporated. Be careful not to overmix.
Reserve ¼ cup of the dark chocolate chips and ¼ cup of the toffee bits for topping. Add the remaining dark chocolate chips, mini chocolate chips, and toffee bits to the dough. Mix on low speed just until evenly distributed.
Using a 3-tablespoon cookie scoop, portion the dough directly onto the prepared baking sheets, leaving about 2 inches between each cookie.
Press a few of the reserved dark chocolate chips and toffee bits onto the tops of each dough ball. This gives the cookies that bakery-style appearance with visible pools of chocolate and toffee.
Bake for 10–12 minutes, or until the edges are lightly golden brown and the centers still look slightly underbaked. The cookies will continue to set as they cool.
Immediately sprinkle the warm cookies with flaky sea salt.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.