Preheat the oven to 375 degrees F and line cookie sheets with parchment paper. Using an electric mixer, cream the butter and sugar together until light and fluffy, 3-5 minutes. Then scrape the bowl and beat in the egg and sour cream until smooth.
Turn the mixer on low and add the vanilla, salt, baking powder and baking soda. Then slowly add the flour, and beat until just combined.
Use a small 2-teaspoon scoop to drop the cookie dough onto the baking sheets in a uniform fashion. (You could roll then into little balls if you're really particular.) Space them 2 1/2 inches apart. Bake for 8 minutes, until the edges are just barely golden. Then cool completely on the baking sheets.
For the Chocolate Ganache Filling:
Place the Nutella and chocolate chips in a microwave safe bowl. Microwave for 1 minute, then stir well so the chocolate chips melt into the Nutella. Scoop the filling into a thick plastic bag and snip the corner off to make a quick piping bag.
Flip half of the cookies over and pipe 1 to 1 1/2 teaspoons of chocolate ganache filling in the center of the turned cookies. Then top each one with another cookie and dust with cocoa powder or powdered sugar to garnish. Store in an airtight container.