1. Preheat oven to 350°. Butter two 9-inch round cake pans. Line bottoms of pans with cooking parchment. In a heavy 1- to 2-quart pan over low heat, stir brown sugar, butter, and 3 tablespoons whipping cream until melted and blended, about 5 minutes. Pour half the brown sugar mixture into each of the cake pans. Sprinkle 3/4 cup chopped pecans evenly into pans.
2. In a bowl, with a spoon, beat eggs, granulated sugar, and oil until well blended. Stir in pumpkin and 1/2 teaspoon vanilla. In a bowl, whisk together flour, baking powder, pumpkin pie spice, baking soda, and salt to blend. Whisk dry ingredients into pumpkin mixture until well blended. Pour half the batter into each of the pans; smooth top.
3. Bake until a toothpick inserted in center of cakes comes out clean, 30 to 35 minutes. Let cool in pans on racks about 5 minutes, then invert onto racks and remove pans and paper. Let cool completely, about 1 1/2 hours.
4. Up to 6 hours before serving, in a bowl, with an electric mixer on high speed, beat remaining 1 3/4 cups whipping cream until soft peaks form. On low speed, beat in powdered sugar and remaining 1/2 teaspoon vanilla just until blended. Set one cake layer, pecan praline side up, on a serving platter. Spread two-thirds of the whipped cream mixture over the top. Set second layer, praline side up, on top. Cover with remaining whipped cream mixture. Sprinkle with remaining 1/4 cup chopped pecans.