Preheat the oven to 350˚F. Butter a 9-inch springform pan and lightly dust with flour. Set aside.
In a standing mixer fitted with a paddle attachment or using an electric hand mixer, cream the butter and sugar together until pale and fluffy, approximately 3 – 5 minutes. Add the eggs and beat on medium speed until fully incorporated.
In a separate bowl, sift together the flour, baking powder, cinnamon and salt. In another bowl, mix together the buttermilk and vanilla. Add the dry ingredients to the creamed butter in 3 additions, alternating with the buttermilk mixture and beating in between each addition, until just combined. Using a rubber spatula, fold in the apple cubes.
In a separate bowl, use your hands or a pastry cutter to combine all of the streusel ingredients together until a crumbly mixture forms.
Pour half of the batter into the prepared baking pan. Sprinkle with half of the streusel. Top with the second half of batter and then sprinkle the remaining streusel evenly over the top. Bake in the preheated oven for 50-60 minutes or until a toothpick inserted into the center comes out clean and the cake springs back when touched.
Transfer the cake pan to a wire rack and allow to cool for 10 minutes. Carefully remove the cake from the pan and place back on the wire rack to cool completely before serving.