Soften butter for the cake layer to room temperature or microwave it at 50% power for 10 seconds at a time until soft enough to press easily.
Bring the egg to room temperature by leaving it out for at least 3 hours or placing it in warm (50–60°C) water for 15 minutes.
Grease a 6-inch fixed-base round cake pan with butter, line the bottom with parchment paper, and butter the parchment as well.
Sift the flour and baking powder for the cake layer, then set aside.
Preheat the oven to 180°C (350°F) and position the oven rack in the lower middle.
Wash and dry the rhubarb, then cut it into 1 cm cubes. In a large mixing bowl, combine rhubarb and sugar with a spatula. Add corn starch and melted butter, mixing until the rhubarb is evenly coated.
Spread the rhubarb mixture in an even layer in the bottom of the prepared cake pan and press gently with a spatula to ensure it’s compact.
In another bowl, add softened butter and sugar, and beat at low speed with an electric mixer until the butter is fluffy and pale.
Add the room temperature egg and vanilla extract, beating again at low speed until evenly combined.
Add all dry ingredients at once and beat at low speed until just combined, being careful not to overmix.
Pour the batter over the rhubarb, spreading it evenly with a spatula.
In a separate bowl, mix all ingredients for the almond crumble, then pinch the mixture into small crumbs and sprinkle evenly over the cake batter.
Bake for 40–45 minutes until the top is lightly browned and the cake is fully cooked. Test with a skewer to ensure no wet batter clings when removed.
Cool in the pan for 30 minutes, then invert the cake onto a plate while still warm.
Serve with Chantilly cream (whipped cream with 10% sugar by weight), if desired.
The corn starch in the rhubarb layer helps thicken the mixture during baking for easier unmolding, so don’t omit it.
Butter and egg for the cake must be at room temperature. Softened butter whips better, and cold eggs may cause the mixture to separate.
Bake until a skewer inserted in the center comes out clean; overbaking can dry out the cake.
For added flavor, serve with Chantilly cream by whipping heavy cream with 10% of its weight in sugar.
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