Spray (or oil) an 8” square pan with cooking oil, line bottom with parchment paper and spray the paper.
Peel the rhubarb, slice any thick stalks in half so that they are all pretty much the same thickness. Cut the stalks into four inch pieces. Save any small bits or trimmings. Toss the four inch pieces in a bowl with ½ cup of white sugar and ¼ cup brown sugar. Layer the stalks in the bottom of the pan in a decorative pattern. Top with any remaining sugar.
Dice the rhubarb trimmings and reserve.
In a medium bowl, stir together the flour, baking powder and salt.
In another beat the butter with remaining sugar and orange zest until light and fluffy, about 5 minutes. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla and yogurt. Gradually add the flour and mix well. Stir in the diced rhubarb.
Spread the cake batter evenly over the rhubarb in the pan.
Bake until cake springs back from the pan and a toothpick inserted into the center comes out clean.
Cool in the pan for about 15 minutes then carefully invert the cake onto platter and peel away the parchment paper.
Recipe Notes
This recipe can be used for many other fruits. Try it with fresh Summer peaches or the apples of Autumn. For very sweet fruits cut the sugar in the base down by 1/4 cup