Combine 2 tbsp espresso powder with 2 tbsp boiling water and stir to dissolve.
Using a mixer with the paddle attachment, cream the butter, granulated sugar and confectioners sugar on medium-high until light and fluffy, about 2minutes. Add the egg yolks, mixing well after each addition. Add espresso, vanilla and salt and combine.
On low speed, mix in the flour. Stop the mixer when you still see a few streaks of flour in the dough, then mix the rest by hand.
Transfer the dough to the counter and split the into two pieces. Form each half into a disc, wrap each with plastic wrap, and place in the refrigerator for 2 hours.
Preheat oven to 325°F and line your baking sheets with parchment paper. Remove one disc from the fridge and let soften on the counter for about 10-15 minutes. Unwrap the dough and place it between 2 sheets of parchment paper. Roll out to a thickness about 1/4 inch
Use a 2" cookie cutter to cut out cookies. Cut them as close together as possible. To prevent sticking. Dip the edges of the cutter into some flour. Repeat with 2nd disk. If the dough becomes too soft or warm, just pop it back in the fridge for a couple minutes.
Place the cookies on the prepared sheets about 2 inches apart. Bake for 12-15 minutes, until the edges are just beginning to turn golden. Let the cookies set for 5 minutes before moving them to cooling racks.
To make the ganache, place the chopped chocolate into a heat-safe bowl. heat the heavy cream in a saucepan over medium heat and bring to a simmer. Pour the cream over the chocolate and let it to melt for for 2 minutes. Add the espresso powder and whisk the chocolate and cream until smooth and creamy.
Place the uncovered ganache in the refrigerator to thicken. Let it chill for 30-40 minutes, stirring occasionally.
Spread a small amount of ganache over half of the cookies, then make sandwiches. If the chocolate ever becomes too stiff, microwave on low for 10 seconds at a time, stirring in between each, until you get the desired consistency.