Preheat oven to 350 degrees. Grease a 9-inch springform pan with nonstick spray and line the bottom with a parchment round; set aside.
In a large bowl, combine the eggs and sugar. Use an electric mixer to mix until light and creamy; about 2 minutes. Add in the softened butter and continue mixing until well combined. Then, mix in the milk and vanilla.
In a separate bowl, whisk together the flour, baking powder, and salt. Pour the dry mixture into the wet and stir together just until combined. Use a rubber spatula to scape down the sides of the bowl to ensure all of the ingredients are well combined.
Pour the batter into the prepared pan and smooth the top with a spatula. Then, use a spoon to dollop the Nutella all around the top of the cake batter. Use a skewer or butter knife to swirl Nutella around the top of the cake.
Place on center rack in preheated oven and bake for about 30-35 minutes, or until a toothpick inserted in the cake comes out clean.
Let the cake cool on a wire rack for 15 minutes, then remove collar and let cool completely.
Notes
Make sure the eggs, butter, and milk are all room temperature before starting. This will ensure you have a smooth batter.
If you don't have a springform pan, you can make this in a 9 inch square baking pan or a cake pan with tall sides.
Store leftover cooled cake in an airtight container at room temperature for two days or refrigerate for a week. You can also freeze this cake in an airtight container covered with plastic wrap for up to a month.