Preheat the oven to 350F. Line an 8” square pan with parchment paper, or grease the pan.
Add the streusel topping to a medium-size mixing bowl. Use a pastry cutter (or two knives) to cut the butter into the ingredients and combine the ingredients, until the mixture resembles coarse crumbs. Set aside.
Using a mixer, cream together the butter and sugar in a large mixing bowl. Add the eggs one at a time, beating well after each addition.
Add the sour cream and vanilla. Beat well to combine the ingredients with the butter mixture.
Add the dry ingredients to the bowl. Gently stir the dry ingredients into the wet ingredients with a wooden spoon or rubber spatula. Stir until just combined - do not overmix.
Spread the batter into the prepared pan. Top with the streusel.
Bake for 45-50 minutes**, until a toothpick inserted in the center of the cake comes out clean.
Let the cake cool before cutting and serving. This cake makes the best leftovers.
Notes
*I used organic cane sugar. Coconut sugar or sucanat will also work. Stick with the weight measurement, if possible, when using a different sugar. **Helen tested this recipe using a glass pan. Her bake time was 25-30 minutes. I used a parchment-lined ceramic pan (linked above, which is actually 9"). My bake time was 45-50 (closer to 50). The bake time will depend on the pan used to make the cake. Darker pans require a longer bake time. Keep this in mind while baking your cake.