To make the lemon curd, whisk the sugar and the cornflour in a saucepan. Add the lemon juice, coconut milk and turmeric and whisk to get a lump-free mixture.
place the saucepan on low to medium heat and continue to whisk vigorously (ensure there are no lumps) for at least 5-7 minutes till the mixture thickens to a glossy, custard-like consistency. Remove from the flame, cool and store in the fridge. The curd will continue to thicken as it chills.
Preheat the oven to 176°C. Lightly grease and line a 9" round springform pan or an 8" square cake pan with baking parchment. Leave some parchment overhanging on 2 sides of the square pan to easily lift the cake out.
Whisk all the dry ingredients, except the sugar, together in a large bowl.
In another bowl, whisk together the buttermilk, oil, sugar, lemon zest and juice.
Pour the wet mixture into the dry one and gently mix until the batter is smooth without any lumps. Do not overmix.
Pour the batter into the prepared tin, drop spoonfuls of lemon curd curd all over, gently swirl with a skewer, leaving about ½ an inch of batter around the edges.
Bake the cake for 30-35 mins till the top is golden and the lemon curd has set. Cover the cake with foil midway through the baking if the top browns too fast.
Cool the cake in the tin for 10 mins before gently lifting it out or un-molding it. Cool on a wire rack before slicing.