Preheat your oven to 350°F. Grease an 8-inch springform pan and line the base with parchment paper. Set it aside.

In a bowl, mix the dry ingredients for the cinnamon streusel: the brown sugar, flour, cinnamon, and nutmeg. Pour in the melted butter and vanilla, then stir until evenly moistened. Set this aside.

In a large mixing bowl, cream the 4 tablespoons of butter with the 1/2 cup granulated sugar until fluffy. Beat in the egg, then mix in the vanilla and sour cream until well combined.

In another bowl, combine 1 cup of flour, baking powder, baking soda, and salt. Gradually add this dry blend to the butter mixture, stirring until just mixed.

Spread half of the batter in the base of the prepared pan, which will create a thin layer. Distribute the diced apples on top, followed by about 1 cup of the streusel mixture. Add the remaining batter on top of the crumbs, and finish with the leftover streusel.

Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean. Before carefully removing the springform pan ring, run a thin knife around the edges.

To make the glaze, whisk the powdered sugar with apple cider until smooth and drizzle it over the cooled cake.

Enjoy this lovely coffee cake with a cup of coffee or tea! It pairs wonderfully with some fresh fruit on the side.