Preheat oven to 350°F and grease a 9-inch cake pan or cast iron skillet.
In a large bowl, beat butter and sugar until fluffy.
Add egg and vanilla; mix until smooth.
Stir in milk.
Whisk together flour, baking powder, salt, and spices in another bowl.
Fold dry ingredients into wet until just combined.
Stir in applesauce.
Spread batter in prepared pan.
Arrange apple slices on top and press gently.
Mix streusel ingredients until crumbly and sprinkle over apples.
Bake for 40–45 minutes or until a toothpick comes out mostly clean.
Cool at least 10 minutes before slicing.
