Preheat oven to 350°F. Remove the tart shells from the wrapping and puncture the bottom and sides of the dough with a fork. Bake for 12-15 minutes until lightly browned. Set aside to cool.
In a large microwavable bowl, whisk together the sugar, milk, egg yolks, flour, cocoa powder, and salt until well mixed. Whisk until the flour and cocoa powder are incorporated. This can take 2-3 minutes.
Microwave on high for 2 minutes and then whisk the mixture. Repeat microwaving for 2-minute intervals and whisking between each interval until the mixture thickens. It will take 6-8 minutes total time.
Scatter the sliced butter in the bowl and add the vanilla. Whisk until the butter melts. Pour the mixture into the prepared tart shells. Refrigerate until completely cool, about 2 hours.
Top with whipped topping and a sprinkling of pecans. Serve chilled.
Notes
Sugar Amount
The recipe originally called for 2 cups of sugar in the pie filling. I reduced the sugar to 1 1/2 cups. You may prefer to add the full 2 cups for sweeter tarts or reduce it further for less sweet tarts.
Pie Instead of Tarts
You can make this dessert in a pre-baked pie shell instead of tart shells.
Yield
This recipe makes 12-16 three-inch tarts, depending on how much filling you put into each.