Preheat oven to 350°F (top/bottom heat) or 325°F (convection oven). Grease and flour a bundt pan.
In a bowl, cream together the butter, sugar, and vanilla extract until light and fluffy. Now, beat in the eggs one at a time.
In a separate bowl, combine the flour, baking powder, and salt. Alternately add the flour mixture and milk to the butter mixture, mixing until smooth.
Transfer half of the batter to another bowl. Stir in the Nutella and cocoa powder into one half and the vanilla extract into the other half.
Spoon alternating dollops of the dark and light batters into the greased Bundt pan. Swirl the batters together with a fork or knife to create a marble effect.
Bake for approximately 55-60 minutes, or until a wooden skewer inserted into the center comes out clean. Allow to cool for at least 1-2 hours before removing from the pan.
For the glaze, melt the chocolate. Stir in the Nutella and oil. Glaze the cake and give it time to set before serving.
