Preheat and Prep: Preheat your oven to 350°F (175°C). Spray an 8 or 9-inch round or square baking pan with nonstick cooking spray.
Prepare the Cake Batter: In a large bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
2 c. unbleached all-purpose flour, 1 tsp baking powder, ½ tsp baking soda, ½ tsp salt, 1 tsp ground cinnamon, ¼ tsp ground nutmeg
Cream the Butter and Sugars: In a separate large bowl, using a hand mixer or stand mixer fitted with the paddle attachment, beat the softened butter, granulated sugar, and brown sugar on medium speed until light and fluffy, about 2-3 minutes.
½ c. salted butter, ¾ c. granulated sugar, ¼ c. light brown sugar
Add Wet Ingredients: Add eggs, vanilla extract, and sour cream and mix until just combined.
2 large eggs, 1 tsp vanilla extract, ½ c. sour cream
Combine Wet and Dry Ingredients: Add the dry ingredients to the wet mixture, mixing on low speed until just incorporated. Do not overmix. Fold in the grated zucchini with a spatula until evenly distributed.
1 c. zucchini
Make the Streusel Topping: In a medium bowl, combine the flour, brown sugar, cinnamon, and salt. Stir in the melted butter until the mixture forms coarse crumbs. If using nuts, fold them in. Set aside.
¾ c. unbleached all-purpose flour, ½ c. light brown sugar, 1 tsp ground cinnamon, ¼ tsp salt, 6 tbps salted butter, ½ c. walnuts or pecans
Assemble the Cake: Spread the batter evenly into the prepared pan. Sprinkle the streusel topping evenly over the batter, covering the entire surface.
Bake: Bake for 45-55 minutes (depending on pan size), or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool and Serve: Allow the cake to cool in the pan on a wire rack for 15 minutes before slicing. Best served warm.
Notes
Yellow Squash Swap: For yellow squash substitution, always squeeze out excess moisture due to higher water content.
To Serve: Enjoy warm with a small pat of butter for an indulgent touch.
Make it a Dessert: Make a simple glaze with 1 cup of powdered sugar (115g) and 1 to 2 tablespoons of milk. Drizzle over slightly cooled cake.