Prepare the Cake: Fill a large roasting pan with hot water to a depth of 2 inches; place on rack in lower third of oven. Preheat oven to 350°F. Generously coat a 14-cup light-colored Bundt pan with cooking spray.

Beat butter, granulated sugar, and brown sugar in bowl of a heavy-duty stand mixer on medium speed until light and fluffy, about 3 minutes. Add egg; beat just until blended. Sift together flour, pumpkin pie spice, baking powder, baking soda, and salt in a bowl. Whisk together pumpkin, buttermilk, and vanilla in a separate bowl. Add flour mixture to butter mixture alternately with pumpkin mixture, beginning and ending with flour mixture, beating on low speed after each addition (batter will be thick).

Evenly pour 1⁄4 cup of the cajeta into prepared Bundt pan. Gently spoon batter over cajeta; smooth top of batter. Set aside.

Prepare the Flan: Place condensed milk, evaporated milk, cream cheese, eggs, and vanilla in a blender. Process on high speed until completely combined, about 30 seconds, stopping to scrape down sides as needed. Pour mixture over batter in Bundt pan. Cover loosely with aluminum foil.

Carefully remove roasting pan with hot water from preheated oven. (Water should be steaming when removed from oven.) Gently place Bundt pan in prepared roasting pan, and return to oven. Bake at 350°F until a wooden pick inserted in center comes out clean, 1 hour and 30 minutes to 1 hour and 45 minutes, rotating Bundt pan halfway through bake time.

Remove Bundt pan from roasting pan; transfer to a wire rack to cool until slightly warm, about 45 minutes. Gently invert onto a rimmed serving plate. Spoon remaining 1⁄4 cup cajeta over top; sprinkle with pecans. Serve additional cajeta on the side.