Beat 1 cup softened unsalted butter with 1/2 cup granulated sugar and 1 cup packed brown sugar in a large bowl using an electric mixer on medium speed until light, fluffy, and creamy, about 3-4 minutes.
Mix in 2 large eggs, one at a time, beating well after each addition. Stir in 1 teaspoon vanilla extract.
In a separate bowl, whisk together 3 cups all-purpose flour, 1/2 cup cocoa powder, 1 teaspoon baking powder, and 1/2 teaspoon salt.
Gradually add the dry ingredients to the wet butter mixture, mixing on low speed or folding with a spatula until just combined. Do not overmix.
Gently fold 1/2 cup chopped hazelnuts into the dough evenly.
Wrap dough in plastic wrap and chill for at least 30 minutes.
Preheat oven to 350°F (175°C). Line baking sheets with parchment paper or silicone mats.
Portion out dough balls about 1 inch in diameter using a spoon or cookie scoop. Place on baking sheet spaced apart. Press thumb or back of spoon into center to create indentation.
Fill each indentation with about 1 teaspoon chocolate hazelnut spread, avoiding overfilling.
Bake for 12-14 minutes until edges are set but centers look slightly soft. Cool on baking sheet 5 minutes, then transfer to wire rack to cool completely.