To make the topping, combine the brown sugar, flour, and cinnamon in a small bowl. Add the butter and work it into the mixture until crumbly. Set aside.

Beat the brown sugar and softened butter together in a separate mixing bowl. Add the egg and mix until well combined, then stir in the milk.

In another bowl, whisk together the flour, baking powder, and salt. Add the dry ingredients to the wet ingredients and stir just until everything is incorporated.

Spread half of the batter into the prepared pan. Arrange half of the apples over the batter and sprinkle with half of the topping.

Spread the remaining batter over the apples, then layer on the remaining apples and topping.

Bake for 45–50 minutes, until a toothpick inserted into the cake comes out clean.

Allow the coffee cake to cool slightly before serving.