In a large bowl, whisk pumpkin and eggs together. Add brown sugar, oil and buttermilk. Whisk to combine.
In a separate bowl, whisk the flour, baking powder, baking soda and spices together. Slowly pour the dry ingredients into the pumpkin mixture and stir to combine.
Fold in the raisins and pecans.
Coat a 9x13 baking pan with nonstick cooking spray. Pour the batter into the pan. Bake 35 minutes. Cool completely before frosting.
To make the frosting, beat the cream cheese, butter and vanilla together in a stand mixer. (Alternatively, use a handheld electric mixer). With the mixer on low speed, add the confectioner's sugar. Increase mixer speed and whip the frosting until fluffy.