Combine butter and sugar in a large mixing bowl. Pour scalded milk over it and stir until butter is melted. Set aside - this needs to cool down a little.
In a small bowl, combine packet of yeast and 1-2 tablespoons warm water (about 100°F) and stir until yeast is dissolved.
Add yeast to butter mixture.
Add flour, egg yolks, salt & lemon rind. Mix until thoroughly combined, no need to knead (hehe)
Cover bowl and let dough rise for about 2 hours in a warm, draft free area.
Divide dough in half. Save other half, covered and refrigerated, for another occasion.
Preheat oven to 350°
Press dough into bottom of a greased 9"x13" baking dish.
Onto the rhubarb. Slice it and toss it with 1/2 cup sugar. Spoon evenly on top of dough.
In a mixing bowl, whisk together sour cream, egg yolks, 1/4 cup sugar and vanilla extract.
Spoon custard mixture over top of cake.
Bake for 35 - 40 minutes. Toothpick inserted near center should come out clean and custard should be set.
Cool on wire rack.
Top with whipped cream and cinnamon if you'd like. Enjoy :)
I store this Rhubarb Kuchen covered in the refrigerator. Let sit at room temp for about half an hour before serving.