01 - Whisk together the gluten-free flour blend, baking powder, and sea salt in a medium mixing bowl. Set aside.
02 - Using an electric mixer, beat softened butter with granulated sugar in a large bowl until the mixture is light and fluffy, about 2 to 3 minutes.
03 - Add the egg and pure vanilla extract to the butter mixture. Continue to beat until thoroughly blended.
04 - Gradually add the dry ingredients to the wet mixture while mixing on low speed until a smooth, cohesive dough forms.
05 - In a small microwave-safe bowl, dissolve instant espresso powder in hot water. Add the chopped dark chocolate and microwave in 15-second intervals, stirring until fully melted and smooth. Let cool slightly.
06 - Divide the dough evenly in half. Gently fold the chocolate-espresso mixture into one portion, mixing just enough to achieve a marbled effect.
07 - On a sheet of parchment paper, pat the plain dough into a rectangle approximately 8 x 6 inches. Pat the marbled chocolate-espresso dough into a slightly smaller rectangle and lay it on top of the first layer. Roll up tightly, as for a jelly roll, to form a spiral.
08 - Wrap the dough log securely in parchment paper and refrigerate for at least 30 minutes until the dough is firm.
09 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
10 - Using a sharp knife, slice the dough log into 1/4-inch thick rounds and space them 2 inches apart on the prepared baking sheets.
11 - Bake in the preheated oven for 10 to 12 minutes, or until the edges are lightly golden.
12 - Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before serving.