Preheat the oven to 325 degrees. Grease and flour a large bundt pan.
In the bowl of your stand mixer, cream the butter and 3/4 cup of the peanut butter until smooth. Beat in the sugar, mixing until light and airy, around 5 minutes. Add the eggs, one at a time, mixing well between each addition. Stir in the vanilla.
Mix in the mashed banana on low.
Add the flour and salt to the mixer, all at once, and mix just until incorporated.
Pour half of the batter into the prepared bundt pan. Drop heaping spoonfuls of Nutella over the top of the batter, about 1/2 a cup.
Using a butter knife, swirl the Nutella all throughout the cake to get a marbled look.
Pour remaining batter over top of Nutella.
Bake the cake for 60 - 75 minutes or until the cake is golden brown and a tester comes out clean.
Allow cake to cool in the pan for 20 minutes, then remove to a wire rack to cool completely.
Heat remaining Nutella and peanut butter in the microwave in 10 second intervals to soften it up enough to drizzle. Drizzle on top of cake.