For the most consistent results, use a kitchen scale and the weight measurements provided for the ingredients. If you don't have a kitchen scale, be sure to measure the flour with the spoon and level method.
Preheat the oven to 350F (non-convection/not fan-assisted). Grease 4 ramekins (see Note 2 below for sizing suggestions) and set them onto a baking sheet. Set aside.
In a small bowl, whisk together the flour, baking powder and salt. Set aside.
In a large bowl with an electric mixer or the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar at medium speed until light and fluffy, about 2 minutes. Beat in the eggs, one at a time, beating well after each addition. Add the vanilla and mix in.
With the mixer on low, add the flour mixture to the batter, alternately with the milk, mixing until well combined.
Divide the batter between the ramekins. (The cake batter should only fill the ramekins by 1/3-12 full.) Spoon the rhubarb over the cake batter, dividing between the ramekins.
Place the ramekins (on top of the baking sheet )into the preheated oven and bake for 25 minutes.
Meanwhile, prepare the custard by adding the custard ingredients to a small bowl. Whisk together well.
When the cakes have baked for 25 minutes, remove them from the oven. Spoon the custard mixture over the top of the cakes, dividing it between the ramekins.
Return the cakes to the oven and bake for another 15 minutes, or until the custard is set and the cake part tests clean when tested with a cake tester.
Remove from the oven. Enjoy hot or warm, or refrigerate to enjoy later. Rewarm in the microwave or covered with foil in a 350F oven until warmed.
Notes
Note 1: If you are a bit short of rhubarb or if you just love the flavour combination, add some chopped fresh strawberries to the rhubarb to make up the difference.Note 2: My ramekins shown in the photos are 3 1/2 inches in diameter and 2 1/4 inches tall. They hold 1 1/4 cups of water filled right to the top. Mine were filled right to the top by the time they baked! A slightly larger ramekin would give a bit more breathing room. Alternatively, or if your ramekins are smaller, simply increase the yield and divide the ingredients between 5 or 6 smaller ramekins.Tips!You may wonder if the rhubarb will be too tart, as it doesn't have any sugar on it. I can assure you that with the sweet cake and the sweetened custard, it is perfectly balanced and not too tart. In fact, I love that the rhubarb's tart flavour really shines in this one! That said, if you prefer a bit more sweetness, consider adding a few chopped strawberries to the rhubarb for sweetness.You can make these cakes as one cake by baking in an 8x8-inch baking pan. The baking time should be extended to about 30 minutes for the first bake and about 20 minutes for the second bake.Be sure to read the notes above this Recipe Card for more tips on making this recipe. You'll also find step-by-step photos that you might find helpful.