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Preheat the oven to 350ºF. Spray a 9-inch deep dish pie plate with nonstick spray.
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To make the brownie filling: combine the butter and chocolate chips in a small saucepan. Heat over medium-low heat, stirring often, until melted and smooth. Stir in the light brown sugar. Remove from the heat and whisk in the eggs, one at a time, the vanilla, flour, cocoa, baking powder, and salt. Pour about 1/2 the batter into the bottom of the pie plate and spread evenly. Bake until shiny and just starting to set, about 20 minutes. Cool 10 minutes.
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Meanwhile, make the pumpkin filling: in a medium bowl, whisk the pumpkin, heavy cream, egg, sugar, flour, pumpkin pie spice, and salt.
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Pour the pumpkin filling into the pie plate over the brownie layer. Whisk 1 tablespoon of water into the remaining brownie batter. With a spoon, drop scoops of brownie batter over the pumpkin batter. Use a butter knife to gently swirl the brownie batter into the pumpkin. (If it seems like the brownie batter is sinking too much, leave some of the scoops on the surface unswirled.)
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Bake until the pumpkin is set and the brownie swirls are shiny, 45 to 50 minutes. Transfer to a rack and cool completely. (The pie can be made ahead and refrigerated for 2 days.) Bring to room temperature before serving.