Combine the hazelnut flour, all-purpose flour, sugar and salt in a food processor. Pulse a few times to combine. Add the softened butter and process until the dough comes together and is the flour is fully hydrated by the butter. Divide the dough into four parts, and roll each into a log about ½ inch thick. Wrap each log in plastic, then pop in the freezer for 15 minutes, or refrigerate for two hours.

Preheat the oven to 325 and line two cookie sheets with parchment paper.

Cut the dough into equal sizes, and roll into balls about the size of a large marble. If using sanding sugar, roll the balls in the sugar. Place on the cookie sheets, about 2 inches apart, and bake for 12-14 minutes.

Let the cookies cool completely.

Melt the chocolate in a double boiler, or in short 15-20 increments in the microwave, stirring between each, until just melted. Spread a bit of chocolate on the flat side of one of the cookies and make a sandwich with another cookie. Repeat until all the cookies are used.

Cookies keep at room temperature for a week.