Preheat the oven to 350 degrees F (175 degrees C). Line 2 baking sheets with silicone baking mats or aluminum foil, shiny side down.

Beat melted butter, white sugar, and brown sugar together using an electric mixer in a large bowl. Add eggs one at a time, beating after each addition. Beat in vanilla extract. Stir oats, pecans, and baking powder into the batter.

Drop batter onto the prepared sheets, 1 teaspoon at a time, spaced 3 inches apart; batter will spread out very thinly while baking.

Bake in the preheated oven, watching very carefully, until edges just start to brown, 6 to 8 minutes. Keep cookies on the baking sheets; they will finish baking as they cool and harden. Cool completely before removing from the baking sheets, at least 10 minutes.

Choose 2 cookies that are the same size. Spread chocolate-hazelnut spread very thinly on 1 cookie and sandwich the other on top. Continue until all the cookies are paired and filled.