Preheat the oven to 350 degrees. Bake a pie crust or buy a pre-baked pie crust. Melt the butter in a double boiler. Add the chocolate and let it melt (5 to 7 minutes) and whisk until smooth. Set aside to cool slightly.

Using an electric mixer at medium speed, beat the cream cheese in a large bowl and gradually add 1 1/4 cups of sugar. Beat the eggs, one at a time. Then, mix in the pumpkin puree, vanilla extract, spices, and salt until the filling is well combined. Pour a little less than half of the filling into a bowl and mix with the melted chocolate until smooth.

Pour the chocolate filling into the pie crust and shake gently to even it out. Bake the tart on the middle rack of the oven for 20 minutes. Then, let it cool on a rack for 15 minutes. Spoon the pumpkin filling over the chocolate layer and shake to even it out. Return the tart to the oven and bake for about 35-40 minutes. Once done, the filling will have puffed at the edges but not enough to create large cracks. Place the tart on the cooling rack until the fillings have set, about 30-45 minutes. Mix the sour cream and 1/4 cup of sugar in a small bowl. Stir the mixture over very low heat for about 2 minutes, until it’s smooth and lightly warm. Pour slowly over the last layer and tilt the mold to spread the filling evenly. Let the tart cool completely, then cover it with aluminum foil and refrigerate for at least 4 hours before serving.