Preheat the oven to 350ºF. Combine all of the shortbread ingredients in a food processor until it resembles coarse meal.
Press the mixture into a 9x13 inch baking pan. Bake in the preheated oven for around 20 minutes, or until the edges are lightly browned. Set aside.
Meanwhile, place the butter and chocolate in a microwave safe bowl. Heat in the microwave in 30 second intervals, mixing until smooth.
Mix in the sugar. Add in the eggs 1 at a time, mixing well after each addition. Mix in the vanilla and Nutella until fully incorporated. Mix in the flour until fully combined.
Pour the brownie batter over the cooked shortbread and spread evenly. Bake for 35-40 minutes, or until a toothpick inserted in the centre comes out clean. Remove from the oven and allow to cool completely before cutting into squares.