Preheat your oven to 350°F (175°C). Grease a 9-inch baking pan.
In a bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt.
Beat softened butter and sugar until light and fluffy. Add eggs one at a time, then vanilla.
Alternate adding dry ingredients and sour cream to the butter mixture, mixing until just combined.
Fold in chopped apples and spread the batter into the prepared pan.
For the crumb topping, combine flour, cinnamon, salt, cold butter, brown sugar, and granulated sugar. Mix until crumbly and sprinkle over the batter.
Bake for 35-40 minutes or until a toothpick comes out clean.
For the drizzle, melt butter in a saucepan. Add brown sugar, cinnamon, milk, and salt. Stir until smooth. Drizzle over the cooled cake.