In a medium heavy-bottomed saucepan, combine the sugar, water, and corn syrup. Gently stir to moisten all the sugar, then stop stirring.

Set the pan over medium heat and cook undisturbed for 5–10 minutes, until the mixture turns deep amber. Meanwhile, warm the heavy cream in the microwave for about 1 minute.

Once the caramel is a rich copper color, remove from heat and carefully pour in the warm cream.
Caution: The mixture will bubble up rapidly. Pour slowly and whisk constantly.

Whisk until smooth. Stir in the vanilla, salt, and butter. Return to low heat briefly if needed to smooth it out.

Let cool slightly, then transfer to a heat-safe jar and refrigerate until ready to assemble.
The caramel will thicken as it cools.

In a large bowl, whisk together the flour, espresso powder, cocoa powder, baking soda, and salt. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter, granulated sugar, and brown sugar on medium-high speed for about 3 minutes, until creamy and fully combined. Scrape down the sides and bottom of the bowl as needed.

Beat in the egg and vanilla extract until fully incorporated.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until a thick dough forms.

Line large baking sheets with parchment paper or silicone baking mats. Roll the dough into small balls, about 1 tablespoon cookie scoop or 30g each, and place them on the prepared baking sheets. Flatten each ball into discs about 1.5 inches in diameter, using the flat bottom of a measuring cup or drinking cup. Be sure to keep the size consistent for even sandwiching later.

Place the baking sheet(s) in the refrigerator for 15 minutes to help the cookies hold their shape during baking.

While the dough chills, preheat your oven to 350°F (177°C).

Bake for 8–10 minutes, or until the edges are set and the tops begin to crack slightly. They should have a dome on them. While the cookies are still remove them from the oven and take a flat bottom like a spatula or the bottom of a measuring cup and flatten them to ¼ inch thick. Let them cool for atleast 5 minutes before transferring them to a wire rack to cool completely.

In a small microwavable bowl, heat the cream until its bubbly and hot. Add the espresso powder and mix until combined. Set aside to completely cool.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed for about 2 minutes until creamy and smooth.

Scrape down the sides of the bowl and beat for another 2 minutes.

With the mixer on low speed, gradually add the powdered sugar and the cooled espresso heavy cream mixture. Once combined, add the vanilla extract and a pinch of salt.

Increase the speed to medium and beat for 1-2 minutes until smooth and fluffy.

If the frosting is too runny, place in the fridge for 15 minutes to set up a little.

Make sure the cookies are completely cooled before assembling.

Pipe a thick rim of espresso buttercream around the edge of half of the cookies to create a well for the caramel. The rim should be sturdy but not overly thick.

Fill the frosting well with the prepared caramel sauce, being careful not to overfill.

Top each filled cookie with an unfrosted cookie, pressing gently to create a sandwich.

Serve and enjoy! Store any leftovers in an airtight container at room temperature for up to 3 days or in the fridge for up to a week.