Take your butter, eggs, and sour cream out of the refrigerator at least 45 minutes before you start. Room temperature ingredients are not optional here. Cold butter will not cream properly and the batter will look curdled. Preheat the oven to 325 degrees F. Grease your 10-cup Bundt pan very thoroughly with softened butter or baking spray, making sure to get into every crevice. Dust lightly with flour and tap out the excess.

In a medium bowl, whisk together the flour, baking soda, baking powder, cinnamon, nutmeg, and salt. Set aside. Peel and dice your apples into small cubes, about 1/2 inch. Pat them dry with a paper towel and set aside separately.

In a large bowl with a hand mixer or in the bowl of a stand mixer, beat the butter and sugar together on medium speed for 3 to 4 minutes until pale, fluffy, and noticeably lighter in color. Scrape down the bowl. Add the eggs one at a time, beating well after each addition. Beat in the vanilla extract.

Reduce the mixer to low speed. Add the flour mixture in three additions, alternating with the sour cream, beginning and ending with the flour. Mix just until combined after each addition. Do not overmix once the flour goes in or the cake will be tough. Scrape the bowl and make sure no pockets of flour are hiding at the bottom.

Fold in the diced apple by hand with a rubber spatula. Spoon and spread the batter evenly into the prepared Bundt pan. Tap the pan gently on the counter a couple of times to settle the batter and release any air pockets.

Bake at 325 degrees F for 60 to 70 minutes. Start checking at the 60-minute mark by inserting a skewer or thin knife into the thickest part of the cake. It should come out clean with no wet batter, though a few moist crumbs are fine. The top will be deep golden and the edges will have pulled slightly away from the pan.

Let the cake cool in the pan on a wire rack for exactly 15 minutes. Then place the rack over the pan and flip the cake out in one confident motion. Let it cool on the rack for another 20 to 30 minutes before glazing. You want it warm but not hot.

Make the glaze by whisking together the sifted powdered sugar, maple syrup, and milk until smooth and pourable. It should drizzle off a spoon in a thin stream. If it is too thick, add milk half a teaspoon at a time. Drizzle it over the warm cake and let it run down the sides. It will soak in slightly as the cake cools, which is exactly what you want.