Line a 9x9” pan with foil and spray with cooking spray (for easy removal and cutting of bars). Preheat oven to 350°F.
Prepare the crust by slowly mixing all the ingredients in a stand mixer fitted with the paddle attachment or in a large bowl using hand mixer. The mixture will be crumbly and if you are using a hand mixer you may need to use your hands to break up any large chunks of butter. Press the crust in the prepared pan and bake for 15 minutes.
Make the filling: Heat a frying pan over low heat. Melt the butter, then add the zucchini, sugar, and cinnamon. Cook, stirring often, for 4-5 minutes until the zucchini just looks like it’s getting translucent around the edges. Drain and let sit until the crust is done.
Make the topping while the zucchini is cooking: you can use the same bowl you used for the crust. Use a stand or a hand mixer to cream the butter and sugar. Add the flour, oats, and salt and mix slowly. The mixture will be somewhat crumbly.
Right before the crust comes out of the oven, drain the zucchini again. Remove the hot crust from the oven. Sprinkle the zucchini carefully on top (make sure they are well drained first - do not add any of the extra liquid!), then sprinkle with the topping. Bake for an additional 25-30 minutes, until the top starts to get golden brown.
Cool completely in pan before slicing into bars and serving. Serve with caramel sauce and/or ice cream or whipped cream.
Store bars in the refrigerator in an airtight container for up to 3 days, or freeze them for up to one month.