Preheat the oven to 350 degrees. Spray a bundt cake pan liberally with cooking spray.
In a mixing bowl, stir together the vanilla cake mix, 2 eggs, pumpkin, and pumpkin pie spice until fully combined. Spread into the bottom of the prepared pan.
In a mixing bowl, whisk together 3 eggs, 1/3 cup oil, 1/3 cup water, and 1 tablespoon vanilla. Add the chocolate cake mix and stir well to combine. Gently pour over the top of the pumpkin layer. Bake for 45 minutes or until a toothpick inserted into the center of the cake comes out clean. Let the cake cool for 30 minutes in the pan then invert onto a wire cooling rack to finish cooling completely.
In a small saucepan, combine the cocoa powder, 6 tablespoons water, and Brown Swerve. Cook, stirring, over medium-high heat until very warm and smooth. Stir in 1 teaspoon vanilla. Remove from the heat and stir in the butter, 1 tablespoon at a time, stirring until the butter is melted before adding the next tablespoon. Stir until all butter is melted and the sauce is smooth and glossy. Let cool a little before drizzling over the top of the cake.