Preheat your oven to 350°F (175°C). Grease and line a 9x13-inch baking pan with parchment paper.
In a large bowl, cream together butter and sugar until light and fluffy.
Beat in eggs, one at a time, then stir in sour cream and vanilla extract.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Spread half of the batter into the prepared pan. Dollop Nutella over the batter and swirl with a knife.
Sprinkle blueberries and chopped hazelnuts evenly over the Nutella layer.
Top with the remaining batter and spread evenly.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.