Preheat the oven to 350°F and grease a 9x13-inch baking dish with butter.
In a large bowl, whisk the pumpkin puree, eggs, heavy cream, powdered sweetener, pumpkin pie spice, vanilla and salt until completely smooth. Pour into the baking dish and bake for 15 minutes so the surface begins to set.
While the pumpkin layer bakes, whisk together the almond flour, coconut flour, granulated sweetener, baking powder and salt. Rub in the cold butter with your fingertips until the mixture forms coarse, clumpy crumbs, then stir in the chopped pecans.
Carefully remove the dish from the oven. Scatter the crumb topping evenly over the partially set pumpkin layer, covering it all the way to the edges.
Return to the oven and bake for 30 to 35 minutes more, until the topping is golden brown and the center barely jiggles when the dish is gently shaken.
Cool at room temperature for 1 hour, then chill for at least 2 hours so the pumpkin layer turns creamy and sliceable. Serve in squares with a spoonful of whipped cream.