Grease a round 8-inch cake pan with shortening. Place a circle of wax paper in the bottom and grease it as well.
Press rhubarb evenly into bottom of the pan. Sprinkle 1/3 cup sugar over rhubarb followed by the orange zest.
In a large bowl, mix butter, sour cream and sugar together. Mix in eggs until smooth and then stir in flour until combined. Pour batter into pan.
Bake for 35-40 minutes until toothpick comes out clean.
Immediately loosen cake by running a knife along the sides of the pan. Remove cake by inverting pan over a plate or cake platter*. (If any rhubarb gets stuck to the pan quickly scrap it off and spread on the top of the cake.)
Let the cake cool and then serve. (Garnish with more orange zest if desired.)
Notes
*I like to place my platter upside down on the cake pan, hold them firmly together while wearing hot pads, flip them so now the platter is sitting right side up on the table with the cake pan upside down, give the pan a few taps, and the cake falls out perfectly in the center of the platter.Recipe Source: Garnish & Glaze Original Recipe