Preheat the oven to 325 F/ 165 C. Line a metal 9x13 baking pan with parchment paper and set it aside.
In a medium bowl, mix together the flour, brown sugar, granulated sugar, cinnamon, and melted butter until dense crumbs form. Chill in the fridge.
Use the large holes of a cheese grater to grate three red apples into a large bowl. Squeeze out all of the juice and place them into a separate bowl.
Measure 1 ½ packed cups (282 grams) of the grated apples. Then measure out ¾ cup (195 grams) of the apple juice from the bowl. Discard any extra juice and apples. If you are short, grate an extra apple (or two) until you have enough.
In a large mixing bowl, whisk together the oil and dark brown sugar. Then whisk in the eggs, vanilla, measured grated apples, and measured apple juice.
Fold in the flour, cinnamon, baking soda, and salt until just combined.
Pour the cake batter into the prepared pan and evenly sprinkle the crumb mixture on top. Bake for 30-40 minutes. To test for doneness, insert a toothpick into the center; it should come out clean but slightly sticky, without any wet batter clinging to it. Allow the pan to cool on a wire rack while preparing the glaze.
Whisk together the powdered sugar, milk, and vanilla extract until smooth. For a thicker glaze, use more powdered sugar. Drizzle the glaze over the cinnamon apple cake then cut it into slices and enjoy!