Rhubarb upside-down cake

I made this last weekend, took this photo on my phone, devoured the cake with a friend (the same night we ate the mushroom tarts, as a matter of fact), and completely forgot about it until right now, though I'm not sure how that happened. It's really good. I used this recipe as a jumping-off point, which has a great, roughly 1:1 fruit:cake ratio, and tweaked it a bit to my taste. I like almond with tart fruit, so added some almond extract and switched out ground almonds for some of the flour, used lemon instead of orange, and cardamom instead of mace.

Rhubarb Upside-Down Cake
adapted from Recipezaar

Base
2 T. butter, melted
1/2 c. sugar
juice of one lemon
4 stalks rhubarb, chopped

Cake
3/4 c. flour
1/4 c. ground almonds
1/2 t. baking powder
pinch of salt
1/2 t. cardamom
1/4 c. butter
1/2 c. sugar
1 egg
zest of one lemon
1 t. almond extract
2/3 c. plain yogurt

Preheat oven to 350F. Combine the base ingredients in an 8" square pan, making sure all of the rhubarb pieces are coated, and set aside.
Combine dry ingredients.
Cream butter and sugar until fluffy, beat in egg, almond extract, and rind.
Mix in the flour mixture alternately with the yogurt, making three additions of each.
Spoon batter over rhubarb, spreading right to the edge of the pan, and bake for 35–40 minutes, until top springs back when lightly pressed.
Let cool for 15 minutes, then run a butter knife around the edge to loosen and invert onto a plate or platter.