Pour the 3 cups (720 mL) cider into a saucepan. Simmer over medium heat, stirring occasionally, until reduced to 1 cup (240 mL), approximately 20–30 minutes or longer. Carefully check the volume with a heatproof measuring jug. If reduced below 1 cup without scorching, add water to make exactly 1 cup. Cool to room temperature, approximately 20 minutes or as needed. Do not use hot reduction in the batter.

Heat the oven to 350°F with a rack in the center. Whisk the flour, baking powder, baking soda, cake cinnamon, nutmeg, and salt in a bowl.

Beat the softened butter, 1 cup granulated sugar, and brown sugar with a mixer on medium speed until lighter and fluffy, about 3–4 minutes. Scrape the bowl. Beat in the eggs one at a time, then mix in the oil and vanilla.

In a separate bowl or jug, whisk the cooled 1 cup cider reduction with the sour cream. On low speed, add the dry mixture to the butter mixture in three additions, alternating with the cider mixture in two additions. Start and end with dry ingredients. Mix just until combined, scraping the bowl, then fold briefly with a spatula to incorporate any remaining flour.

Thoroughly coat a 12-cup Bundt pan with baking spray containing flour, including the center tube and all grooves. Fill immediately. Spoon in the batter and smooth the top. Do not fill more than three-quarters full; reserve any unexpected excess for a separate bake rather than overfilling.

Bake at 350°F for approximately 50–65 minutes, beginning checks at 50 minutes. A tester inserted into the thickest part should come out clean or with a few moist crumbs, with no wet batter. The surface should look set and spring back gently. If not done, continue baking and check again in a few minutes.

Rest the cake in its pan on a rack for 15 minutes. Gently loosen the exposed edges and center tube if needed with a thin flexible spatula. Wearing oven mitts, place a cooling rack over the pan, invert them together, and carefully lift the pan off the cake.

Set the warm unmolded cake and rack over a tray. Mix the coating’s 1/2 cup sugar with its 2 teaspoons cinnamon. Brush the top and sides with the 4 tablespoons melted butter, then sprinkle and gently press on the cinnamon sugar. Let stand at least 10 minutes before slicing, or cool longer for cleaner slices.