1) To make the dough: Mix the sugar, butter, salt, and baking powder.
2) Add the flour; mix until crumbly.
3) In a separate bowl, whisk together the egg yolks, egg, and vanilla.
4) Add to the flour mixture and stir until the dough is cohesive.
5) Shape into a disk, wrap in plastic, and refrigerate for at least 1 hour.
6) To make the filling: Combine 3 cups of the rhubarb, the sugar, vanilla, and ClearJel or flour in a heavy saucepan.
7) Simmer over medium-low heat for 10 to 15 minutes, or until the rhubarb breaks down. Transfer to a bowl, stir in the remaining rhubarb, and cool.
8) To make the topping: Combine the flour, sugar, and salt.
9) Work in the butter to make pea-sized crumbs.
10) Add the pistachios, stirring to combine. Place in the refrigerator.
11) Preheat the oven to 375°F. Place 8 English muffin rings on a parchment-lined baking sheet.
12) To assemble and bake the tartlets: Roll the dough 1/8" thick. Cut 8 1/2" to 9" circles, and fill the rings with the dough, trimming the edges.
13) Fill each tartlet with 1/4 cup of the rhubarb. Sprinkle on the topping.
14) Bake the tartlets for 35 minutes, until the crust and topping are golden and the filling is bubbly.
15) Remove from the oven, and serve warm.
Yield: 8 individual tarts.