Preheat the Oven: Set the oven to 350°F (175°C) and line baking sheets with parchment paper.
Mix the Cookie Dough: Cream butter and sugar until fluffy. Add the egg and vanilla, beating until fully combined.
Combine Dry Ingredients: In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt.
Make the Dough: Gradually add dry ingredients to the wet mixture, stirring until a dough forms.
Shape the Cookies: Roll the dough into 1-inch balls and place them on the baking sheets, flattening slightly.
Bake: Bake for 12-14 minutes or until edges are set. Let the cookies cool on a rack.
Prepare the Ganache: Heat heavy cream and espresso in a saucepan until it just starts to simmer.
Melt the Chocolate: Pour the hot cream over chocolate chips and let sit for 2 minutes. Whisk until smooth.
Add Vanilla: Stir in vanilla extract for added flavor.
Assemble the Cookies: Spread ganache on the flat side of a cookie, then sandwich with another cookie.
Set and Serve: Let the ganache set before serving, or dive in if you’re impatient (no judgment).