Preheat your oven to 325 degrees F. Thoroughly grease every groove and the center tube of your bundt pan with softened butter using a pastry brush or your fingers, making sure no spot is missed. Dust with flour, tapping out the excess. Set the pan aside on a baking sheet.

In a large mixing bowl, whisk together the flour, baking soda, salt, cinnamon, nutmeg, and allspice until evenly combined. Make a well in the center.

Pour the vegetable oil, granulated sugar, and brown sugar into the well. Add the eggs, vanilla, and sour cream. Using a rubber spatula, stir everything together from the center outward until the batter is smooth and no dry flour streaks remain. Do not overmix.

Add the diced apples to the batter and fold them in gently with the spatula, just until distributed. The batter will be thick, and that is normal.

Scrape the batter into the prepared bundt pan and spread it into an even layer. Bake for 55 to 65 minutes. Start checking at 55 minutes by inserting a wooden skewer into the thickest part of the cake. When it comes out with just a few moist crumbs and no wet batter, the cake is done.

Let the cake cool in the pan on a wire rack for exactly 15 minutes. Do not wait longer or the steam will cause sticking. Place the wire rack over the pan, flip them together in one confident motion, and lift the pan away. Let the cake cool on the rack for another 20 minutes until just warm but not hot.

While the cake finishes cooling, make the glaze. Whisk the sifted powdered sugar, maple syrup, and milk together in a small bowl until completely smooth. The glaze should fall off the whisk in a thick ribbon. If it is too thick, add milk a teaspoon at a time.

Place the still-warm cake on its serving plate or stand. Pour the glaze slowly over the top, letting it run down the ridges naturally. Allow the glaze to set for about 10 minutes before slicing and serving.