In a medium bowl, whisk together flour, baking powder, and salt. Set aside.

In a large mixing bowl, cream together butter and sugar until light and fluffy.

Beat in the egg and vanilla extract until combined.

Dissolve instant coffee granules in hot water, then stir into half of the dough.

Add cocoa powder to the coffee dough and mix until smooth.

Leave the remaining half of the dough plain.

Roll each dough half into a rectangle of similar size.

Place the chocolate-coffee dough over the plain dough and gently roll them together into a log.

Wrap the log in plastic wrap and refrigerate for at least 1 hour, until firm.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

Slice chilled dough into 1/4-inch thick rounds and arrange on prepared sheets.

Bake for 10–12 minutes, or until edges are lightly golden.

Cool on wire racks before serving.