Position rack in center of oven. Preheat oven to 350°F/180°C.

For the Topping: Take a 10-inch (25 cm) ovenproof skillet, preferably nonstick, and arrange the rhubarb across the bottom, cutting to fit. You can use a few pieces of rhubarb to make up one whole “stripe” that goes from one side to the other (use image to help understand this). Just pack them in there like logs, then remove and set aside. This step is just to get them cut to the right size.

With the now empty pan, begin to melt butter over medium heat. Stir in sugar and continue to cook for a minute or two until it is well combined, bubbly and the sugar is melting. Add rhubarb, packing the pieces in there; the pan will be full and you will have a few creating a second layer, most likely. Cook gently for about 3 to 5 minutes or just until rhubarb is beginning to soften and the whole mixture is juicy. Set aside.

For the Cake: Stir the milk a lemon juice together in a measuring cup and set aside for about 5 minutes to thicken.

Whisk the flour, baking powder, cinnamon and salt together in a small bowl to aerate and combine; set aside.

Beat the butter and sugar together in a large mixing bowl with an electric mixer until soft and creamy. Beat in the vanilla, then beat in the eggs one at a time, beating well after each addition and scraping bowl down as needed with a silicone spatula. Add the flour mixture and the soured milk and beat just until combined. The batter will be thick. Dollop it out using two spoons on top of the rhubarb. Then very gently spread it out without dislodging or disrupting the rhubarb. This I best accomplished with a small-dampened offset icing spatula, and a very light touch.

Bake for about 30 to 35 minutes or until a toothpick inserted into the cake comes out clean. Use your small icing spatula to run around the edge of the cake, then place a large flat platter on top of cake, and carefully but swiftly invert everything. Wear your hot mitts as needed. If any rhubarb and topping sticks to the pan, simply remove it and put it into place on the cake. We like this cake warm, but room temperature is okay too. It should be eaten the day it is made.