Rhubarb Upside Down Cake

INGREDIENTS

4 tbsp

unsalted butter, very soft

1/2 cup

all-purpose flour

1/4 cup

sugar

1/4 tsp

salt

1 1/2 sticks

(12 tablespoons,) unsalted butter, room temperature, plus more for buttering pan

1 lb

rhubarb, trimmed and cut on a very sharp diagonal about ½ inch thick

1 3/4 cups

sugar

2 tsp

vanilla bean paste, extract (or vanilla beans scraped from a pod.)

1 1/2 cups

all-purpose flour

1 1/2 tsp

baking powder

1 1/2 tsp

salt

zest of 1 lemon

large eggs, room temperature

1 cup

sour cream