I am so glad I finally managed to take a photo of this delicious cake before gobbling it all up, so I can post the recipe for you. My folks came up for a visit this weekend, and what better excuse do you need to make a glorious, buttery, rhubarb upside-down cake? None, I say. 
So far I’ve made this cake three times, exactly as written below. I adapted it from an English recipe, for US ingredients and also for what I happen to have on hand. I use plain kefir instead of milk to impart a nice tanginess to cut the richness of the cake. You can use buttermilk instead, or just plain milk (though this will change the flavor of the cake somewhat). The topping is perfectly sweet and tart, and the whole shebang really melts in your mouth. As you can see in the photos I prefer mine with just a little drizzle of cream, but you can serve this cake with whipped cream, ice cream, or just on its own. It’s delicious no matter what.
Note: for the 3 Tbs of butter, I actually just use what’s left of a whole stick after I cut off the 1/3 cup butter needed for the cake. To get technical, 1/3 cup butter = 5 and 1/3 Tbs from a standard US stick of butter. So that means I use the remaining 3 and 2/3 Tbs butter for the topping.
Rhubarb Upside-Down Cake
Adapted from The English Kitchen
Makes one 9 inch round cake, which serves 6-8
Topping:
3 Tablespoons butter
1/4 cup granulated sugar
1/4 cup demerara sugar (raw sugar)
1/4 cup light brown sugar, unpacked
2 cups chopped rhubarb, fresh or frozen (if frozen, thaw and drain off the liquid before using)
Cake:
1/3 cup butter, softened
1/2 cup granulated sugar
1 extra large egg
1 tsp vanilla
1 cup all-purpose flour
1 1/2 tsp baking powder
1/4 tsp salt
1/2 cup plain kefir
Regular or whipped cream, for serving (optional)
Oven @350F.
Topping: Melt the butter in a small microwaveable bowl. Pour butter into the bottom of a 9 inch round cake tin and swirl to coat the bottom completely. Sprinkle the sugars evenly over the butter. Spread the rhubarb pieces evenly on top of the sugar.
Cake: Cream butter and sugar until fluffy. Add egg and vanilla, beating well. In a separate medium bowl, stir together the flour, baking powder, and salt. Add the flour to the butter mixture in thirds, alternately with the kefir (starting and ending with the flour mixture). Carefully spread the cake batter over the rhubarb in the cake pan.
Bake for 50 to 55 minutes until a toothpick inserted in center of cake comes out clean. Cool in the pan for 10 minutes, then run a knife around the edge of the pan and invert cake onto a plate. Serve warm, with cream or whipped cream if desired.
