That recipe only calls for 1 teaspoon of liquid espresso; there is no way that the espresso is the liquid that is making it hard to work with.  The liquid in the recipe is coming from the eggs.

That said, if you wanted to substitute that 1tsp, I'd use 1/2 tsp [instant espresso powder][1].  Since the mix of instant espresso powder for making a fake espresso is [almost 1:1 by volume][2], you can cut it in half any time to use dry.

Do not use instant coffee, it will not work.  The goal of adding espresso to a recipe like the biscotti is to make the chocolate taste "richer" by adding additional roasted and bitter notes. Most instant coffee lacks these flavors.  There are exceptions, such as Cafe Bustelo, that are specifically formulated to have a darker-roasted flavor and would work as well as espresso powder.  But anything described as "crystals" is probably better simply omitted.



  [1]: https://www.thekitchn.com/what-is-espresso-powder-how-to-use-41586
  [2]: https://www.medagliadoro.com/recipes/hot-drinks/medaglia-doro-espresso