Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round or square baking pan.
In a medium bowl, whisk together 2 cups flour, baking powder, baking soda, salt, and 1 teaspoon cinnamon. Set aside.
In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract, lemon zest, and lemon juice.
Alternately add the dry flour mixture and sour cream to the wet ingredients, beginning and ending with the flour mixture. Mix until just combined.
Fold in the grated zucchini until evenly distributed.
In a small bowl, combine the brown sugar, 1/3 cup flour, 1/2 teaspoon cinnamon, cold cubed butter, and chopped walnuts (if using). Use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs.
Pour the batter into the prepared pan and spread evenly.
Sprinkle the streusel topping evenly over the batter.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan on a wire rack for at least 20 minutes before slicing and serving.