Preheat oven to 350 degrees F. In the bowl of a stand mixer fitted with a paddle attachment, cream together butter and confectioners' sugar until smooth.
In another bowl, whisk together espresso powder, cocoa powder, flour, and salt. Add flour dry mixture to creamed butter mixture. Mix until almost combined. Add vanilla and continue to mix until there are no longer any dry streaks of flour.
Wrap dough in plastic wrap and let chill in the fridge for 30 minutes. Roll dough out on a lightly floured surface to a thickness of about 1/4-inch. Punch out rounds using a 1-1/2 inch circle cutter. Gather dough scrapes and press together. Roll to 1/4-inch thickness and continue to punch out additional rounds. Place rounds on a parchment lined baking sheet, spacing rounds about an inch apart.
Bake for 15 minutes. Let cool in pan for 2-3 minutes before transferring to wire rack to cool completely.
Peanut Butter Filling: In the bowl of a stand mixer fitted with a paddle attachment, cream together peanut butter and butter until combined. Add confectioners' sugar and salt. Mix until filling is smooth.
To assemble: Pair together espresso shortbread cookies. Place about 2 teaspoons of peanut butter filling on half of the cookies. Sandwich with remaining cookies. If desired, drizzle melted white chocolate over the tops of the cookie sandwiches and sprinkle with cacao nibs.