Preheat oven to 350˚F with the rack in the middle. Grease a 9-inch round cake pan with butter & parchment to line the bottom.
Assemble the streusel topping. In a medium bowl mix together the flour, oats, sugar and salt. Rub the butter into the flour mixture until the streusel resembles coarse breadcrumbs. Store in the fridge until ready to use.
For the cake: Using a stand mixer & paddle attachment, cream the butter and granulated sugar on medium-high speed until light & fluffy. Add the vanilla, then beat in the eggs one at a time. In a medium bowl, whisk together the flour, baking powder, cinnamon, ginger and salt. With a rubber spatula, fold the dry ingredients into the wet along with the milk.
Transfer the batter to the prepared pan. Layer the apples over the top, then sprinkle the streusel over the apples.
Bake the cake for 60 to 70 minutes. The top should be golden brown and crisp. A toothpick inserted into the middle of the cake should come out virtually crumb free. Remove from the oven, let cake cool a bit before flipping it onto a cooling rack or cake platter.
Serving Size 1 slice