Preheat oven to 350° F. Toast the pecans (you can also toast the ones you’ll need for the buttercream at the same time). Spread the pecans evenly over a sheet pan and toast in the oven for 3-7 minutes, or until they smell nutty and have slightly browned. Be sure to keep a close eye on them so they don’t burn! Set aside to cool.
Prepare three 8-inch round cake pans by spraying with nonstick spray and lining the bottom with parchment paper. Spray on top of the parchment paper.
In a medium bowl sift together the flour, baking powder, baking soda, salt, and spices. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, mix together the vegetable oil, crushed pineapple, granulated sugar, brown sugar, eggs, and vanilla. Mix on medium speed for about 2 minutes.
Slowly add the dry ingredients to the mixer and mix until just combined. Fold in the shredded carrots and chopped pecans.
Divide the batter evenly between the three 8-inch pans and bake for 23-28 minutes, or until a toothpick inserted in the center comes out with just a couple crumbs stuck to it.
Remove from the oven and let cool in the pans for 10 minutes. Turn cakes out of pans onto a cooling rack until they’ve cooled completely.
Level cake layers, if needed.
If making the layers in advance, wrap completely with plastic wrap and store in the fridge for 1-2 days, or the freezer for 2-3 weeks.