In the bowl of a stand mixer or food processor (you can also use a hand mixer, or your hands) mix together the flour, sugar and hazelnuts. Add the softened butter, vanilla and cooled espresso and mix until the dough comes together. Turn out onto waxed or parchment paper and form into an 8" log. Wrap the log in the paper and twist the ends, smoothing the shape as you go. Chill for at least an hour, or overnight. Pre-heat the oven to 325, and line and baking sheet with parchment. Slice the dough into 1/3 to 1/2 inch slices. Bake in the center of the oven for about 12 minutes. Cool on the pan briefly before finishing on a rack. When the cookies are completely cool, melt the chocolate for about a minute in the microwave stirring until completely smooth, then drizzle or dunk the shortbread.You can just dust them with powdered sugar if you don't want to use the chocolate.