1 1/3 cup all-purpose flour
1/2 tsp baking powder
1/2 tsp salt
1/4 tsp ground nutmeg
1 cup (2 sticks) unsalted butter, softened
1 cup (packed) brown sugar
1 cup granulated sugar
2 large eggs
3 cups old-fashioned rolled oats
1/2 cup roughly chopped hazelnuts
1/2 cup mini chocolate chips
1/4 cup plus 2 tbsp Nutella
1/4 cup heavy cream
Preheat oven to 350 degrees F, with rack set in middle of the oven. Line a 13- by 9-inch baking pan with aluminum foil, leaving a some foil overlapping on each side. Spray lightly with cooking spray.
In a medium bowl, whisk together flour, baking powder, salt and ground nutmeg.
In the bowl of a stand mixer (or in a large bowl with a hand mixer), beat butter, brown sugar, and granulated sugar on medium speed until light and fluffy, 3 to 5 minutes. Add eggs, one a time, beating after each addition.
Turn the mixer to low and gradually add flour mixture. Beat until just combined, about 30 seconds. Stir in oatmeal, chocolate chips and hazelnuts.
Press the mixture into the prepared pan. Bake until the top is turning golden brown and a toothpick inserted in the center comes out just slightly moist, 25 to 30 minutes. Let cool in the pan for 15 minutes. Holding edges of foil, lift the bars out of the pan and let cool completely on wire rack. Cut into squares.
In a small saucepan set over medium-low heat, whisk together Nutella and heavy cream until melted and smooth. Drizzle over bars. Serve.