Directions:

Make the toffee:
Line a baking sheet with parchment paper or a silicone mat.

In a medium nonstick saucepan over medium heat, melt the butter, brown sugar, and salt, stirring with a wooden spoon until evenly incorporated. If there are grainy lumps or if the fat starts to separate, add a couple splashes of water and stir vigorously to emulsify. Continue to cook, stirring constantly, until the mixture reaches 302°F (150°C), about 10-15 minutes.

(Quickly!) pour the toffee onto the prepared baking sheet and spread out slightly with a spatula. Freeze until ready to use, about 30 minutes.

Make the cookie dough:
Add the butter to a medium nonstick saucepan over medium-high heat. Cook, stirring occasionally with a wooden spoon, until the butter turns dark brown and smells nutty. Watch carefully so the butter does not burn. Remove the pot from the heat.
Add an ice cube to the butter and slowly let it melt. It may froth and bubble. Add the remaining 3 ice cubes and gently stir until melted. (Can let butter set to soft in the freezer or fridge but want room temperature before using)

In a large bowl, combine the white granulated, brown sugar, and espresso powder

Add the brown butter into the sugar mixture and mix until combined about 2 mins if using a stand mixer. (Do not overmix)

In a small bowl, whisk together the eggs and vanilla with a fork. Pour into the sugar and butter mixture and mix until glossy and smooth. (Do not overmix, about 2 mins if using stand mixer paddle attachment)

In a large bowl add the flour, salt and baking soda and whisk until evenly distributed

Add the flour mixture by folding with a rubber spatula to incorporate until the dough has no visible flour streaks. Or if using a stand mixer, dump all the flour in at once and mix on low speed until combined (about 2 mins, do not overmix)

Remove the toffee from the freezer (it should be a frozen disc) and break it into chunks that will fit inside a quart or gallon zip-top bag. Double bag the toffee and seal both bags. With a rolling pin, break the toffee into small pieces.

Fold the crushed toffee and chopped chocolate into the dough until evenly distributed. (If using stand mixer, add all of the chocolate and toffee at once and mix until distributed (about 20 seconds or less)

Wrap dough with plastic wrap and place in the fridge to rest and hydrate for minimum of about 1 day.

When ready to bake, preheat the oven to 350°F. Line 2 baking sheet with parchment paper.

Using a large ice cream scoop or a ¼ cup measure, scoop the dough onto the parchment-lined baking sheets. Basically each cookie should be two regular sized cookies together. To get a "craggly" texture pull apart each ball of dough and stick it back together with the torn edges facing out.

Arrange the cookies on the baking sheets, leaving at least 2 inches (5 cm) of space between the cookies and away from the edges of the pan.

Bake until the cookies are golden brown with crispy edges, but are still slightly soft in the center, 10-12 minutes.

Immediately sprinkle the cookies with flaky salt, then let cool completely. The cookies will be very soft at first but will set after they are fully cooled.

Enjoy!