Preheat oven to 350°F. Butter and line your 9" pan with parchment paper. Set aside.

Make the streusel topping by combining flour, sugar, oats, and salt in a medium bowl. Use fork or fingertips to rub the cold butter into the flour mixture until it looks like breadcrumbs. Place in refrigerator until cake is ready.

In a large bowl, cream the butter and sugar together until light and fluffy.

Beat in the eggs one at a time until combined. Then, stir in the vanilla.

In a separate, medium bowl, combine the flour, baking powder, cinnamon, and salt until well mixed. Slowly fold dry materials into the wet mixture. Add milk.

Transfer batter to your cake pan and lay apple slices in an even layer on the top. Cover apple layer with streusel topping. Press down gently with your clean hand.

Bake until top is golden brown and crisp, about 65-75 minutes. Remove from oven and cool slightly before removing from cake pan.

Before serving, dust with powdered sugar.