Prep the Bundt Pan
Grease your fancy shmancy bundt pan thoroughly with softened butter and dust with flour (or almond flour). Tap out the excess.
Whisk the Dry Ingredients
In a separate bowl, combine flour, baking soda, cream of tartar, salt, cinnamon, and nutmeg.
Mix the Wet
Cream butter and both sugars until light and fluffy. Beat in eggs one at a time, then vanilla. Add sour cream and mix until smooth.
Combine
Add dry ingredients gradually, mixing until just combined. Fold in chopped apples.
Layer It Up
Spoon half the batter into the bundt pan, sprinkle half the streusel, then the rest of the batter, then the remaining streusel on top.
Bake
Bake at 350°F for 55–60 minutes, until a toothpick comes out clean.
Cool 15 minutes in the pan before flipping onto a rack.