Gather all ingredients. Preheat the oven to 350 degrees F (175 degrees C). Grease and generously flour an 8-inch square baking dish.
To make the cake: Stir flour, cinnamon, baking soda, and salt together in a bowl.
Beat brown sugar and butter with an electric mixer in a large bowl until light, fluffy, and noticeably lighter in color. Mix in egg, then mix in sour cream, yogurt, and vanilla until well incorporated.
Gradually mix in flour mixture, then fold in apples. Pour batter into the prepared baking dish.
To make the crumb topping: Mix brown sugar, flour, butter, and cinnamon together in a bowl with a fork until crumbly. Sprinkle over cake batter.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 35 to 40 minutes. Remove from the oven and cool on a wire rack.
Serve and enjoy!
