1. Grease a 10 inch bundt cake and preheat oven to 350°F (176°C).
2. To make the walnut streusel, combine the first four ingredients in a small bowl.
3. Add the melted butter and stir until crumbly. Set aside.
4. To make the pineapple walnut topping, use a spoon to drip the melted butter over the bottom of the pan.
5. Sprinkle half of the brown sugar over the bottom of the pan.
6. Place the pineapple slices into the bottom of the pan, evenly spaced. Reserve the pineapple juice from the can (should be about 2/3 cup of juice).
7. In a small bowl, combine the remaining brown sugar, cinnamon and walnuts, then add them to the pan. I filled in the holes of the pineapple, then spread them around the outer edges of the pineapple. Set pan aside.
8. To make the cake batter – In a large mixer bowl, cream the butter and sugar until light and fluffy, 3-4 minutes.
9. Add the sour cream and vanilla extract and mix until well incorporated.
10. Add the eggs one at a time, mixing until well incorporated after each.
11. Combine the flour, baking powder and salt in a medium bowl. Combine milk and pineapple juice in a small measuring cup.
12. Add half of the dry ingredients and mix until combined.
13. Add the pineapple juice mixture and mix until combined, scraping down the sides of the bowl as needed to make sure all is incorporated.
14. Add remaining dry ingredients and mix until well combined.
15. Stir the walnut streusel into the cake batter.
16. Pour the cake batter into the prepared cake pan and spread evenly.
17. Bake for 60-65 minutes, or until a toothpick inserted comes out with a few crumbs.
18. Allow to cool for about 45 minutes (or until cool), then invert onto a serving dish. You can serve it warm or allow it to cool completely and serve it at room temperature. The cake can be refrigerated but is best served warm or at room temperature.