In a saucepan over medium-high heat, combine the ½ cup rhubarb, brown sugar, granulated sugar and vanilla. Add the water and cook, stirring occasionally, until syrup is pink and thickened just a bit, about 15 minutes. Strain through a fine mesh strainer and set aside.
For the Rhubarb Upside Down Cake
Heat oven to 350°F. Add 3 tablespoons of the butter to a 9-inch cake pan. Melt in the oven for about 1 minute, remove and set aside.
Trim the ends of the rhubarb and discard, then slice stalks into 1 or 2-inch pieces. In a bowl toss together with ¾ cup granulated sugar, 1 teaspoon vanilla and cornstarch. Arrange the rhubarb mixture in the bottom of the cake pan with the butter, lining up rhubarb if desired and making sure there are no large gaps (pieces may not fit in a single layer). Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream remaining 8 tablespoon butter and 1 cup granulated sugar until lightened, about 1 minute. Add eggs, and 2 teaspoon vanilla. Beat until fluffy and combined, about 1 minute.
Add the flour, salt, and baking soda and stir slowly until almost combined. Add the milk while the mixer is going on low speed. Increase speed slightly and beat for 10 to 15 seconds to combine. Stir by hand to evenly mix batter then pour over the rhubarb in the pan. Gently spread into an even layer.
Place the pan on a baking sheet and bake, until the cake is golden and a toothpick inserted in the middle comes out with only a few crumbs, 35 to 40 minutes.
Cool cake in pan about 15 minutes, then loosen edges with a knife and invert cake by placing a platter or parchment lined plate over the cake pan and gently flipping it over.
If any rhubarb stayed in the pan, gently remove it and nestle it back onto the cake. Cool cake, then brush with rhubarb syrup before serving if desired.
Serve with rhubarb syrup and whipped cream. Cake can be stored, covered, for up to two days. Syrup can keep in the fridge for up to a week.
Notes
• See tips in the post for extra information• Use extra rhubarb syrup on hot cereal or drizzled on pancakes.