Preheat the oven to 350°F (175°C) and prepare an 8x8 inch baking pan by greasing and flouring it.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy (about 3-4 minutes).

Beat in the eggs one at a time, mixing well after each addition, followed by the vanilla extract.

In a small bowl, mix the yogurt (or sour cream) with fresh lemon juice until smooth.

Gradually add the dry ingredients and the yogurt mixture to the butter mixture, starting and ending with the dry ingredients, mixing until just combined.

Gently fold in the grated zucchini and lemon zest into the batter.

Pour the batter into the prepared baking pan and spread it evenly.

Bake for 30-35 minutes, checking for doneness with a toothpick.

Allow the bars to cool in the pan for 15-20 minutes before transferring to a wire rack.

Once cooled, whisk together powdered sugar with milk or lemon juice until smooth for the glaze.

Drizzle the glaze over the cooled bars and let set before slicing.