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 the all-purpose flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.
In a large bowl, beat the softened unsalted butter and granulated sugar together with an electric mixer on medium speed until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract and lemon zest.
In a small bowl, combine the fresh lemon juice and sour cream.
Alternately add the flour mixture and the lemon juice-sour cream mixture to the butter mixture, beginning and ending with the flour mixture. Mix until just combined.
Fold in the grated zucchini and chopped walnuts (if using) gently until evenly distributed.
Pour the batter into the prepared baking pan and spread it evenly.
Bake in the preheated oven for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan on a wire rack for 15 minutes.
While the cake is cooling, prepare the lemon glaze by whisking together the powdered sugar and 2 tablespoons of fresh lemon juice until smooth.
After 15 minutes, remove the cake from the pan and place it on a wire rack. Drizzle the lemon glaze evenly over the top of the warm cake.
Allow the glaze to set before slicing and serving.