¼cup(57g)salted butter, melted and cooled until just warm
1-2teaspoonsfresh lemon zest, from about 1 lemon
1cup(142g) all-purpose flour
6small/medium rhubarb stalks, about 8-inches long (10-12 ounces)
Preheat the oven to 350 degrees F. Lightly grease a 9-inch round cake pan (you can line the bottom with a parchment circle, if desired).
In a large bowl, whisk together the eggs, egg yolk, and sugar with a wire whisk (don't use a hand mixer - it will incorporate too much air for this cake) until pale and thick, 30 seconds to 1 minute.
In a small bowl, stir together the melted butter, sour cream and lemon zest. Add to the egg mixture and whisk until combined.
Add the flour, baking powder and salt. Mix with a rubber spatula until the batter is smooth. Spread evenly in the prepared pan. Chill the batter for 10 minutes.
Make sure the rhubarb stalks are dried well. If they are over 1/4-inch thick, slice them carefully in half lengthwise. Cut the stalks to fit and arrange over the batter next to each other, covering as much of the batter as you can (but keeping the rhubarb in a single layer). DO NOT PRESS THE RHUBARB INTO THE CAKE BATTER. Lay it very gently on top without pressing at all or else it will be prone to sinking during baking (mine still does each time I make this, but it definitely sinks less if I haven't pressed the rhubarb at all into the cake).
Bake the cake for 45-55 minutes until browned and puffed around the edges. The center may still be slightly soft set but it shouldn't be wet batter. Add additional time, if needed.
Let the cake cool for 10 minutes (or completely) before slicing into pieces. Serve with a dollop of whipped cream or a scoop of ice cream. It's delicious warm, room temperature or chilled.
Rhubarb: using longer pieces of rhubarb makes for a prettier presentation, but it can be a little "stringy" cutting into the cake. Alternately, you can cut the rhubarb into bite-size pieces and scatter those over the top (don't press them into the batter!). They'll most likely sink while baking, but the combination of tart rhubarb in every bite and sweet custard cake is still super delicious. Also, I think you could definitely try other fruits in place of the rhubarb (raspberries or blueberries come to mind!).Frozen Rhubarb: I haven't tried using frozen rhubarb in this recipe. My recommendation would be to probably thaw and pat dry before using.Sinking Fruit: I've made this cake many times, and my rhubarb sinks to some degree every time. It doesn't affect taste at all, so I don't stress about it too much. A few factors that may help prevent sinking: don't overmix the sugar and eggs. It's really important to mix them until pale and thick for the custard texture (you'll notice the color lightening) but too much mixing will incorporate more air which contributes to the rhubarb sinking. Also, using thin stalks of rhubarb (or cutting them in half so they are thinner) will also help.