Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a large mixing bowl, cream together the softened butter and granulated sugar with an electric mixer on medium speed until the mixture is light and fluffy, about 2-3 minutes. Beat in the egg and vanilla extract until just combined.
In a separate medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing on low speed until a soft dough just begins to form. Drizzle in the cooled coffee and mix until the dough is smooth and fully combined.
Scoop the dough by rounded tablespoonfuls and roll into balls. Place them about 2 inches apart on the prepared baking sheets. Bake for 10-12 minutes, or until the tops of the cookies are set. The cookies will be soft. Let them cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.
While the cookies cool, make the mocha filling. Place the chocolate chips in a heatproof bowl. Heat the heavy cream and instant espresso powder in a small saucepan over medium heat until it just begins to simmer. Immediately pour the hot cream over the chocolate chips and let it stand for 2 minutes. Whisk the mixture until it is completely smooth and glossy to create a ganache. Let the ganache cool and thicken for about 20-25 minutes at room temperature, stirring occasionally, until it is a spreadable consistency.
Once the cookies and ganache are completely cooled, spread a generous tablespoon of the mocha filling onto the flat side of one cookie. Top with a second cookie to create a sandwich. Repeat with the remaining cookies and filling.