In a large bowl, whisk together the melted butter and the sugar. Once smooth, whisk in the egg, sour cream/yogurt, milk and vanilla extract. Stir the mixture until it is a smooth batter. Gradually add in the dry ingredients and stir until just combined. Stir in the zucchini and chocolate chips. Spread the batter into the greased pan. Bake for 30 to 35 minutes, then remove from the oven and let cool in the pan for 20 minutes.