Start by pre-heating the oven to 190 C. Prepare a 9 inch cake pan by brushing it on the inside with butter. Keep the pan aside.
Make the topping by combining together the butter, flour, sugar, and salt in a mixing bowl. Once the mixture comes together in crumbs, keep the bowl aside.
In another mixing bowl, toss together the rhubarb and 1/4 cup of sugar, and let this stand for around 3 minutes.
In the prepared cake pan, dot the base with two teaspoons butter, and then add the rhubarb to the base, lining them or scattering them, as you choose. Keep the cake pan aside.
Prepare the cake batter: In a stand mixer or a large mixing bowl with a hand-held mixer, add 1 cup sugar, 100 grams butter, and beat until light and fluffy.
Add 2 eggs to the mixture, beating after each addition.
Add the lemon zest, lemon juice, curd and beat on medium speed until combined.
In a small mixing bowl, add the baking powder and flour, and mix them together with a whisk.
Add the flour and baking powder to the wet ingredients in the bowl, slowly, mixing until just combined. Do not overmix the ingredients.
Transfer the batter to the cake pan, and give the pan a few raps on the counter, so that any trapped air bubbles can escape.
Sprinkle the crumb topping over the batter and set the cake to bake in the oven.
Bake for around 50 -55 minutes at 190 C, until a skewer inserted into the cake comes out clean.
Once the cake it done, let it cool, and then invert it onto a cake platter and serve.