Preheat oven to 350 F. Line the bottom of an 8 or 9 inch springform pan with parchment paper. Spray sides with baking spray. 

In a large bowl, sift the flour, sugar, cocoa powder, baking powder, baking soda, and salt together. Set aside.

In a medium sized bowl, whisk together the eggs with the oil. Whisk in the the vanilla extract, milk and sour cream. Whisk in the melted and slightly cooled Nutella. 

Use a rubber spatula to gradually stir the wet ingredients into the dry ingredients. Stir until just combined, being careful not to over mix. Pour in the hot water and stir until combined.

Pour the batter into the prepared pan and bake 34-39 minutes or until a toothpick comes out clean. Place pan on a wire rack and allow to cool in the pan for 30 minutes then remove from pan and cool on a wire rack. Cool completely before frosting. 

Place the Nutella and softened butter in a heatproof bowl. In a small saucepan on low heat, bring the heavy cream to just boiling then pour over the Nutella and butter. Do not stir. Let stand 5 minutes then stir till smooth. Allow to cool slightly before using.

Beat the peanut butter and room temperature butter on medium. Scrape down the sides and add the vanilla and salt.

With the mixer on low speed, add the powdered sugar and beat for another minute. Add the heavy cream and beat until buttercream is combined and spreadable. 

Use an offset spatula to spread the Nutella ganache over the cake. Let set 15 minutes. Use a spatula to spoon the peanut butter buttercream on top of the Nutella ganache. Use an offset spatula to spread the buttercream overtop.