Preheat oven to 350F and prepare three 6-inch cake pans.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a medium bowl, whisk together the melted butter, brown sugar, granulated sugar, eggs, and vanilla. Whisk in the shredded zucchini.
Pour the wet ingredients into the dry ingredients and mix with rubber spatula until just combined.
Gently fold in the milk with the spatula.
Divide the batter into the three prepared pans.
Bake for 33-35 minutes or until a toothpick inserted in the center of the cake comes out mostly clean with no wet batter.
Lemon Cream Cheese Frosting
In the bowl of a mixer, beat together cream cheese and butter until creamy on medium until smooth and uniform.
Add lemon juice, lemon zest, vanilla extract and mix well.
With the mixer low, slowly add in the confectioners' sugar. Once incorporated, beat the buttercream on medium-high until light and fluffy, 3 - 5 minutes.