Preheat the oven to 350°F (175°C). Grease and flour an 8-inch square baking dish.
STEP 2
In a bowl, stir flour, cinnamon, baking soda, and salt.
STEP 3
Beat brown sugar and butter until fluffy. Mix in egg, sour cream, yogurt, and vanilla. Gradually mix in flour mixture, then fold in apples. Pour batter into the dish.
STEP 4
To make topping: Mix brown sugar, flour, butter, and cinnamon in a bowl until crumbly. Sprinkle over batter.
STEP 5
Bake for 35 to 40 minutes until a toothpick comes out clean. Cool on a wire rack.