INGREDIENTS
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for cake: 1 package white cake mix 1 egg 1 cup light plain, Greek yogurt 1/4 cup butter (1/2 stick), melted 1 tsp vanilla for topping: 1/2 cup flour 3 tsp ground cinnamon 1/2 cup brown sugar 1/2 cup (1 stick) butter, melted 3/4 cup chopped walnuts (o