Preheat the oven to 350°F and grease a Bundt pan with nonstick cooking spray.

Add the walnuts, brown sugar, 1 teaspoon of the zest, and the cinnamon to a food processor and pulse until combined. Cut 1 stick of the butter into tablespoon-sized pieces and add to the food processor along with the 2 tablespoons flour and 2 tablespoons of the Grand Marnier. Pulse to combine. Spoon the walnut mixture into the Bundt pan.

Cream the sugar and the remaining 1 1/2 sticks of butter in the bowl of an electric standing mixer on low for 10 minutes, until light and fluffy. Add the mascarpone and continue to cream one minute longer.

Combine the sour cream, eggs, the remaining 2 teaspoons zest, 3 tablespoons of the orange juice, and 1 tablespoon of the remaining Grand Marnier in a small bowl with a whisk.

Sift 2 cups of the cake flour with the baking powder, baking soda and salt.

Add one third of the egg mixture to the mixer and mix on low until combined. Add one third of the flour mixture and mix on low until combined. Continue two more times, using the remaining egg mixture and flour mixture. Using a rubber spatula, scrape the beater and the bottom of the mixing bowl and fold the ingredients together until well combined.

Spoon the cake batter into the Bundt pan on top of the walnut mixture and lightly spread with a spatula to even.

Bake for 50 minutes, turning the cake midway through cooking. Test the cake for doneness by inserting a toothpick into the cake, avoiding the crack. Only small crumbs should adhere. Remove the cake from the oven and allow to cool in the pan 5 to 10 minutes before inverting onto a cooling rack.

Make an orange glaze by combining the powdered sugar with the remaining 1 tablespoon of Grand Marnier and the remaining 3 tablespoons of orange juice. Drizzle over the warm cake.

Allow the cake to cool completely before serving.