Preheat oven to 350F and grease & flour three 8" cake pans. Line with parchment.
In a medium bowl, whisk together flour, pumpkin pie spice, baking powder, baking soda, and salt.
In a large bowl, whisk together sugars, oil, pumpkin, vanilla, and eggs.
Add dry to wet and mix until just combined.
Divide batter evenly between prepared pans and bake until a toothpick inserted into the center comes out mostly clean (approx. 30-35 mins).
Cool in pans on wire rack for 10mins, then turn out onto wire racks to cool completely.
Line a large baking sheet with parchment.
Heat a large frying pan over medium heat. Add pecans and toast for approx. 3 mins, stirring often.
In a small bowl or measuring cup, combine sugar, cinnamon, water, and vanilla. Stir well.
When the pecans are fragrant, add sugar mixture into the pan. Stir constantly over medium heat for 5-10mins. I used two heatproof spatulas for this.*
Spread onto parchment lined baking sheet to cool. Separate them as best as possible with the spatulas.
Cream Cheese Frosting:
Using a stand mixer or hand mixer beat butter until pale and creamy.
Reduce speed to low. Add powdered sugar 1 cup at a time and mix on low until well blended. Increase speed to medium and beat for 3 minutes. Add vanilla and continue to beat on medium for 1 minute.
Slowly add chilled cream cheese one cube at a time (about 1 tablespoon at a time). Beat well until blended and ensure no lumps of cream cheese remain.
If needed, add cornstarch or meringue powder 1 tablespoon at a time to stiffen the frosting (I didn't but this is an option).
Run mixer on low for a couple of minutes to remove any air bubbles.
Place one layer of cake onto a serving plate or cake stand. Top with 1 cup frosting and spread evenly. Sprinkle 1/3 cup chopped candied pecans over the top of the frosting (reserve the prettiest ones for the top). Repeat with next layer.
Place final cake layer on top and crumb coat the cake. Chill for 30mins.
Use the remaining frosting to cover the outside of the cake. Decorate the top and sides with a swirl if desired (I used a large offset spatula to do this). Decorate with whole and chopped candied pecans.
* The sugar syrup will coat the pecans and eventually turn grainy. You can stop here if you like or keep cooking/stirring until they are caramelized and golden. Watch them carefully as they will burn quickly near the end.Candied Pecan recipe adapted from The Cafe Sucre Farine.