In a medium bowl, whisk together flour, cocoa powder, salt and baking powder until well-combined.

In a large bowl using an electric hand mixer or in the bowl of a stand mixer with the paddle attachment, beat sugar and butter on medium-high speed until light and fluffy, about 3 to 4 minutes.

Beat in egg and vanilla until just combined.

Reduce speed to low; stir in dry ingredients until dough just comes together.

Divide in half; pat each half into a rough disk, then wrap tightly in plastic wrap.

Refrigerate 30 minutes to 1 hour or until dough is still pliable but no longer sticky.

Transfer one chilled dough disk to a large sheet of parchment paper; sprinkle top lightly with flour.

Use a floured rolling pin to roll dough out to approximately ⅛-inch thickness, sprinkling top with just enough flour as needed to keep rolling pin from sticking.

Top dough with another sheet of parchment; transfer to a baking sheet.

Repeat with second dough disk, placing on top of first rolled-out dough on baking sheet (so they are stacked).

Refrigerate dough 1 hour until very firm.

Heat oven to 350°F.

Line two large baking sheets with parchment paper.

Remove chilled dough from refrigerator.

Using a 2-inch cookie cutter, cut cookies out of dough.

Use a smaller cookie cutter to cut out centers from half of cookies.

Transfer to prepared baking sheets, spacing cookies about 1 inch apart.

Sprinkle tops of cut-out (not whole) cookies with sparkling sugar, if desired.

Bake cookies 7 to 10 minutes or until edges are set.

Let cool completely on baking sheets.

Repeat process with remaining cookie dough (keep dough chilled while baking first batch).

Once cookies are fully cooled, spread about 1 teaspoon Nutella on bottom side of whole cookies; top with cut-out cookies to form cookie sandwiches.

Store cookies in an airtight container at room temperature for up to 3 days.