Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.
In a large bowl, cream the butter and sugar together until fluffy.
Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
Gradually add the dry ingredients, alternating with the milk. Mix until just combined.
Fold in chopped apples and walnuts.
Pour the batter into the prepared pan and smooth the top.
For the topping: Mix brown sugar and cinnamon, then sprinkle over the cake.
Bake for 35-40 minutes or until a toothpick comes out clean.
Let cool for 10 minutes in the pan, then transfer to a wire rack to cool completely.