Necessity is the mother of invention. This is what happens when we find fresh rhubarb at the market and rediscover previously made frosting in the fridge. Penuche typically refers to a vanilla fudge candy, but here it is whipped into a frosting. 

1 9-inch tart shell, pre-baked

1 pound rhubarb, washed and cut into 3/4-inch pieces
1 cup sugar
3/4 cup water
1 vanilla bean

1 1/2 cups brown sugar
1/2 cup heavy cream
4 tablespoons butter

Combine rhubarb, sugar, water and vanilla bean and simmer, stirring occasionally, until rhubarb is soft and the mixture is thickened, 10 to 15 minutes. Remove from heat, extract vanilla bean (rinse, dry and toss into your sugar canister to make vanilla sugar); cool. If the mixture is too liquid, strain to remove excess juice. Reserve.

For the penuche, combine brown sugar, cream and butter in a saucepan. Cook over moderate heat, stirring until the sugar dissolves. Allow to cook (without stirring) until caramel reaches 130 degrees on a candy thermometer, 10-15 minutes. Remove from heat, transfer frosting to a large glass bowl, and beat with a mixer until frosting is thick, pale and can be easily spread. (This will make too much frosting, but it can keep, tightly sealed, in the refrigerator for 6 weeks or so.)

Spread frosting on cooled tart shell. Cover with rhubarb mixture and chill. Serve with whipped cream.