0 Preheat the oven to 350F degrees.
1 In the bowl of a standing mixer or with a handheld mixer, cream together the butter and sugar for 4-5 minutes, until light and fluffy.
2 Add the eggs, one at a time, making sure to fully incorporate before adding the next one. Add the vanilla.
3 In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
4 Alternate between adding the flour mixture and the sour cream to the butter/sugar mixture until combined.
5 In a small bowl, combine the brown sugar, walnuts, cinnamon and melted butter, and stir to combine well.
6 Pour half the batter into a greased bundt pan, and smooth down with a spatula as this batter is thick and fluffy.
7 Sprinkle the brown sugar mixture all over the batter in the bundt pan and pat it down lightly.
8 Pour in the remaining batter and smooth down with a spatula.
9 Place the bundt pan in the oven and bake for 45-50 minutes, until a toothpick inserted in the center comes out clean.
10 Remove from the oven, and allow to cool for 15 minutes. Flip cake out of pan into a cooling rack. If necessary, run a sharp knife carefully around the edges of the pan to loosen the cake to flip.
