Preheat oven to 350ºF and spray an 8-inch round cake pan with baking spray.
In a large bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt.
Add oil, nutella, light brown sugar, eggs, vanilla, and milk. Mix on medium speed with a mixer until ingredients are combined and batter is smooth, 1-2 minutes.
Pour cake batter into pan and bake for 28-20 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Let cake cool in pan for 10-15 minutes, then transfer to a wire rack to cool completely.
To make the frosting
Cream butter in the bowl of a stand mixer until smooth and light in color. Add chocolate hazelnut spread and cocoa powder. Mix until combined; scrap sides of bowl with a rubber spatula.
Add powdered sugar and vanilla. Mix on low speed until mostly combined. Add milk and beat on high speed until fluffy and all powdered sugar is incorporated. (If frosting is too thick, add milk one tablespoon at a time until the desired consistency is reached.)
Spread frosting onto the cooled cake, top with garnishes, and serve!