Preheat the oven to 325°F . Grease and flour a bundt pan. Set aside.
To make streusel:
Combine all streusel ingredients in a food processor. Pulse until mixture resembles coarse crumbs.
To make cake:
Combine flour, baking soda and baking powder in a small bowl and gently whisk to combine. Set aside.
Place butter and sugar in a large bowl and mix on medium speed for 2-3 minutes or until creamy. Add eggs and vanilla and mix until incorporated. Add half of flour mixture to butter mixture and mix. Beat in sour cream until incorporated and mix in remaining flour. Batter will be thick.
Pour half of streusel mixture in the bottom of the bundt pan and spread evenly. Add ⅔ of batter on top of streusel and gently level with offset spatula or back of a spoon. Sprinkle remaining streusel evenly over batter and top with remaining ⅓ batter. Smooth top and bake for 50-60 minutes or until toothpick inserted in cake comes out clean. Cool completely in pan and invert on serving tray when ready to eat.
I highly recommend using full fat sour cream and grass-fed or European style butter for this recipe. The higher fat content in both ingredients help keep the cake tender and give it superior flavor compared to reduced fat sour cream and grocery store brand butter. You can also bake this in a 9x13 pan. Make the recipe per the directions and bake at 325°F for 35-45 minutes or until toothpick inserted in center of cake comes out clean. The cake will be thinner than if baked in a bundt pan. If you don't have a food processor, use a pastry blender (or two forks) to work the streusel ingredients together. If you're going this route, make sure you use chopped walnuts and room temperature butter to make incorporating the ingredients a lot easier. An inexpensive oven thermometer is a handy tool to gauge if your oven runs a bit hot or cool. Start testing your cake at the low end of the recommended baking time and adjust time if needed to avoid overbaking. A toothpick inserted in the center of the cake should come out clean. Got leftovers? Freeze individual pieces of this cake tightly wrapped in plastic wrap and/or aluminum foil for up to three months.