Preheat oven to 350 degrees F. Grease and lightly flour a 10-inch fluted tube pan; set aside.
In a large bowl stir together flour, baking powder, pumpkin pie spice, salt, and baking soda; set aside. In another large bowl beat eggs lightly with a fork. Stir in carrots, honey, oil, and buttermilk. Add egg mixture all at once to flour mixture. Stir until combined, fold in the chopped walnuts. Spoon batter into prepared pan, spreading evenly.
Bake for 30 to 35 minutes or until a wooden toothpick inserted near center comes out clean. Cool cake in pan on a wire rack for 10 minutes. Invert cake onto wire rack. Cool completely.
Place cooled cake on a serving platter. Pipe bottom edge of cake with Mascarpone Cream, or pipe or spoon Mascarpone Cream on each serving.
For the Mascarpone Cream:
In a small bowl stir together mascarpone cheese and honey. Gradually stir in enough whipping cream to make a piping consistency.