Chopped rhubarb is mixed into the batter of this tea cake, which is topped with whipped cream and syrupy bits of poached rhubarb.
For the cakes: Preheat oven to 350 degrees. Butter eight 4-by-2 1/2-inch mini loaf pans. Line with parchment cut to fit. Butter parchment. Whisk together flour, salt, baking powder, and baking soda.
Beat butter and sugar with a mixer on medium-high speed until pale and fluffy, about 3 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. Reduce speed to low. Add flour mixture in 2 additions, alternating with sour cream. Stir in rhubarb.
Divide batter among pans. Place on a rimmed baking sheet. Bake until a toothpick comes out clean, about 40 minutes. Remove from sheet, and let cool in pans on a wire rack. (Cakes can be stored in airtight containers for up to 3 days.)
For the rhubarb and syrup: Using the tip of a paring knife, scrape vanilla seeds into a saucepan, reserving pod for another use. Add water and sugar, and bring to a simmer, stirring to dissolve sugar. Remove from heat, and stir in rhubarb. Let cool. Remove rhubarb with a slotted spoon, and reserve. Return liquid to a simmer, and cook until reduced by half, 5 to 8 minutes. Let cool slightly, then return rhubarb to syrup. (Rhubarb can be refrigerated in syrup for up to 1 week; bring to room temperature or gently reheat before serving.)
To serve, spoon a generous amount of whipped cream over each cake, and top with rhubarb and syrup.