1. Grease and line a 35cm x 24cm x 5cm / 13¾" x 9½" x 2" tin. Pre-heat the oven to 160°C
2. Blend the sour cream, bananas, sugars, salt and melted butter in a JTC Omniblend.
3. Add the eggs and blend briefly until incorporated.
4. Add in the flour, baking powder and baking soda and blend briefly until smooth. Don’t over blend it.
5. Divide the mixture between two bowls and in one half, stir in yellow gel food colour.
6. In the other half, gently stir in 150g of Nutella.
7. Grease and line a brownie tin. Pour the cake batters into the tin and use a teaspoon to create a marble effect. Dollop another 100g of Nutella on top of the batter and swirl in.
8. Tap the tray on the work surface three times to remove any air bubbles.
9. Bake for 35 minutes or until a toothpick inserted into the middle comes out clean.
10. Allow to cool in the tin. Cut into squares and store in an airtight container for up to 3 days.