Make dough

Cup of Yum
1. To the bowl of a stand mixer fitted with a paddle attachment (or large mixing bowl with a hand mixer), add the butter and granulated sugar. Beat on medium speed for about 2 minutes.
2. Add the powdered sugar and beat for about 1 minute, then add vanilla and beat until combined.
3. Add the flour, espresso powder, and salt, beating until combined.

Chill dough

1. Lay out a large sheet of plastic wrap on your counter or other work surface. Transfer the dough to the center of the plastic wrap, then wrap the dough up tightly.
2. Chill in the refrigerator for at least 1 hour, or up to 3 days.

Prepare to bake

1. Preheat oven to 350°F and line several baking sheets with parchment paper. Set aside.

Roll and cut the dough

1. Flour your work surface, then unwrap the dough and place it on the work surface. Gently roll the dough out into a rectangle that's about 1/4 inch thick.
2. Use a 2 inch cookie cutter (I used a scalloped round cutter), cut as many cookies out of the dough as possible. Place cut cookies on prepared baking sheet, leaving about an inch in between each cookie.
3. Use your hands to gather the remaining dough and roll it out into another 1/4 inch thick rectangle, cutting as many cookies out as you can and placing them on a prepared baking sheet. Repeat this until you don't have enough dough left to roll and cut.
4. This step is optional, and will depend on how quickly you worked through the rolling/cutting stage, and how warm you keep your home. I like to pop my baking sheets in the refrigerator for about 15 minutes or so, just to make sure they hold their shape. But again, that's optional, so if you feel you don't want to do it, you don't have to - just know they may not keep their shape as well.

Bake and cool

1. Bake in preheated oven for about 10 minutes, until lightly golden on the bottoms and just starting to be golden at the edges.
2. Transfer cookies to wire racks to cool completely.

Make ganache and fill

1. Add the chocolate chips to a bowl and set aside. Add the heavy cream to a different, microwave safe, bowl and microwave for about 1 minute.
2. Pour the hot heavy cream over the chocolate chips and let them sit about a minute. Stir until a smooth and thick chocolate sauce forms.
3. Transfer the ganache to a piping bag fitted with an open star tip (I've linked to the one I used below).
4. Flip the cookies over so the flat side is facing up, then hold the piping bag directly over the top of one cookie. You can pipe any way you like, but I prefer to hold the bag about 3/4 inch above the cookie, and just gently squeeze the bag, lifting as you squeeze, then stop squeezing but keep lifting to finish.
5. Leave a small border, about 1/8 inch, around the edge of the cookie. Top that cookie with another one, gently pushing it down a little to push the ganache to the edges.
6. Repeat with all remaining cookies. Since the ganache was likely still warm, place the sandwich cookies on a plate in the refrigerator for a little bit to fully set.