Preheat your oven to 350°F (175°C). Grease and flour a 12-cup bundt pan (10 inches across) to prevent sticking. If you’re using a different pan size, refer to notes on adjustments.
In a medium bowl, whisk together the cake flour, baking powder, baking soda, cinnamon, and salt. Set this mixture aside.
In another bowl or large measuring cup, combine the sour cream, milk, and vanilla extract. Stir with a fork until blended, then set aside.
Using a stand mixer or hand mixer, beat the softened butter on medium speed until smooth. Gradually add the sugar, continuing to mix until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, ensuring each is fully incorporated before adding the next.
With the mixer on low, alternate adding the flour mixture and the sour cream mixture, beginning and ending with the dry ingredients (three additions of dry, two of wet). Mix just until combined, being careful not to overmix—the batter will be thick.
Gently fold in the diced pears, making sure they are evenly distributed throughout the batter.
Pour the batter into the prepared bundt pan, smoothing the top with the back of a spoon.
Bake at 350°F (175°C) for 45-50 minutes, or until a toothpick inserted into the center comes out clean with just a few crumbs attached. If the top of the cake browns too quickly, cover loosely with aluminum foil around the 40-minute mark. Allow the cake to cool in the pan on a wire rack for 10 minutes before inverting onto a serving plate.