1 ½ cups all-purpose flour
¾ teaspoon baking powder
¼ teaspoon salt
12 tbsp (1 ½ sticks) unsalted butter, softened
1 1/3 cups sugar
4 eggs, room temperature
2 teaspoons pure vanilla extract
1 cup Nutella
1 tablespoon unsweetened natural cocoa
Preheat oven to 325 degrees and adjust oven rack to the middle position. Prepare loaf pan with non-stick baking spray and sugar the inside of the pan, shaking out any excess. Set aside.
Sift 1 ½ cups flour, ¾ tsp baking powder and ¼ tsp salt into a medium bowl and set aside. Place 12 Tbsp butter and 1 1/3 cups sugar in a large mixing bowl. Beat softened butter and sugar with an electric mixer (or stand mixer) until light and fluffy. Add the eggs one at a time until combined, scraping the bowl well after each egg is added. After the last egg, add 2 tsp vanilla and stir. Add the flour mixture a little at a time and stir until just combined, scraping the bowl often.
Measure ½ cup of cake batter into a medium bowl and add 1 cup Nutella and 1 Tbsp cocoa powder. Stir until combined (the mixture will be thick like brownie batter).
Pour approximately 1/3 of the vanilla cake batter into the prepared loaf pan and spread it out flat with a spatula. Spoon or crumble ½ of the chocolate mixture on top of the cake batter. Repeat with half of the remaining vanilla batter and the remaining chocolate. Swirl the cake batter and chocolate with a wooden skewer but do not overmix. Top with remaining vanilla batter and smooth out the top with a spatula.
Place in oven and bake for 1 hour 15 minutes or until a toothpick inserted in the center of the cake comes out with a little crumb. Turn the oven off and allow the cake to sit in the oven for an additional 5 minutes. Remove from oven and place on a rack to cool for at least 30 minutes. Remove cake from pan and allow it to cool right-side up on rack for 1 ½ hours before serving. Enjoy!