01 -
Preheat the oven to 177°C. Grease a 9-inch square baking pan or line it with parchment paper.
02 -
In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon.
03 -
In a large bowl, whisk the melted butter and brown sugar together until smooth. Add the egg, vanilla extract, and sour cream; whisk until fully combined.
04 -
Add the dry ingredients to the wet ingredients and stir until just combined. Be careful not to overmix.
05 -
Fold in the chopped apples gently.
06 -
Pour the batter into the prepared pan and smooth the top for even baking.
07 -
Mix the topping ingredients (granulated sugar, ground cinnamon, and melted butter) in a small bowl and sprinkle evenly over the batter.
08 -
Bake for 35–40 minutes, or until a toothpick inserted in the center comes out clean.
09 -
Let the cake cool in the pan before slicing and serving.