• Toast hazelnuts on a lined baking sheet at 350°F for 5 minutes. You can also toast them on a dry skillet on medium heat until they smell nutty. Stir occasionally while toasting.
• Grind in a food processor until fine. Be careful not to overprocess or else it will turn into hazelnut butter. Combine hazelnut crumbs with flour, baking powder, and salt.
• Using an electric mixer, beat butter and sugar together on medium speed until creamy, about 2 minutes. Add egg and vanilla and beat on medium high until fully incorporated.
• With the mixer on low, add the flour mixture in two additions, combining fully between each addition. Dough may be slightly crumbly, keep running the mixer on low until it comes together. Place dough in fridge and chill for 2 hours.
• Preheat oven to 350°F. Roll out dough to 1/4 inch thick and cut out 1 1/2 inch circles. You can lightly flour your work surface and rolling pin if the dough is sticking. Bake for 9 minutes. Let cool on baking sheet for 5 minutes before transferring to a cooling rack to cool completely.

Make the ganache for the sandwich filling

• Follow instructions for toasting hazelnuts above. This time, grind in a food processor until it’s a creamy paste. It may not be perfectly smooth.
• Place chopped chocolate in a heat safe bowl. Heat heavy cream in a small sauce pan on medium heat until it just starts to bubble. Pour over chopped chocolate. Let sit for 2 minutes, then stir until smooth. Add the hazelnut butter, vanilla, and salt. Put in the fridge to set until solid. When ready to use, let sit at room temperature for one hour. Scoop desired amount of filling between two cookies.