Begin by preheating your oven to 350 degrees F. Grease and flour an 8-inch springform pan (or a 9-inch square baking pan).
Next, prepare your streusel by stirring together the flour, brown sugar, cinnamon, and salt in a medium bowl. Drizzle this mixture with the melted butter and vanilla. Toss together with a fork to combine.
In a large skillet over medium heat, cook the apples in the 2 tablespoons of unsalted butter for 5-7 minutes or until tender. Stir in the brown sugar. Simmer, uncovered, for about 60 seconds. Remove from the heat and set aside.
In a medium bowl combine the flour, sugar, baking powder, baking soda, and salt. Add the melted butter and toss with a fork until the mixture resembles coarse crumbs. Make a well in the center of the mixture. Add the egg, buttermilk, and vanilla. Stir just until moistened. (The batter will be lumpy.)
Spread about 1 cup of the batter into the prepared pan. Spoon the apple filling over the batter in an even layer. Sprinkle with 2/3 cups of the streusel mixture. Drop the remaining batter in small mounds onto the filling. Sprinkle with remaining streusel.
Bake for 40-45 minutes or until a toothpick inserted near the center comes out clean. Cool in the pan on a wire rack for at least 30 minutes.
Meanwhile, make the icing by whisking together the powdered sugar, spices, salt, and enough apple cider to create a drizzling consistency.
Serve warm, drizzled with the icing. (The cake is still delicious served at room temperature.)