Preheat the oven to 325°F (163°C). Grease a 9×5-inch loaf pan with nonstick spray then line the sides with parchment paper to help prevent the sides of the pound cake from burning. Lightly grease the parchment.

In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 1 minute. Add the sugar and beat together for 4-5 full minutes until light and fluffy. Stop and scrape down the sides and bottom of the bowl with a silicone spatula a couple times as needed. Add the sour cream and vanilla, and beat until combined. With the mixer running on medium speed, add the eggs one at a time and beat after each addition.

Pour in the dry ingredients and beat on low just until combined. Do not over-mix. Using a silicone spatula, fold the batter together a few times to ensure there are no pockets of flour or butter at the bottom of the bowl.

Spoon and spread about ⅓ of the cake batter into the prepared loaf pan. Spoon half of the Nutella on top and gently spread it with the back of a spoon, avoiding the edges of the pan. Gently spread another ⅓ of the batter on top, followed by the remaining Nutella. Finish with the remaining batter. Using a knife or skewer, very gently swirl the layers together. The batter is thick, so do your best and avoid over-swirling.

Bake for 65–80 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs and no wet batter. Bake time is a guideline; all ovens vary, so begin checking around 1 hour. If the cake is browning too quickly on top, loosely tent with aluminum foil.

Cool the cake in the pan set on a cooling rack for 1 hour, then remove the cake from the pan and place it directly on the rack to cool completely. Slice and serve.

Cover leftovers tightly and store at room temperature for up to 1 week.