Preheat the oven to 350°F Line 12 muffin cups with cupcake liners. Put the flour, baking powder, salt, and sugar in a large bowl and whisk lightly to aerate and combine.
Mix the egg, yogurt, milk, and melted butter together in a pitcher. Pour them into the dry ingredients and mix lightly, stopping as soon as everything is combined – it's essential not to overmix or you'll get dense, cakey muffins.
Add 2/3 of the lemon curd in 6 or 7 dollops and quickly "marble" it lightly through the mixture (a couple of light stirs is really add that's needed). If your lemon curd is a bit stiff, just add it in little blobs.
Spoon the mixture into the muffin cups, to three-quarters fill them. Bake for about 30 minutes, until golden brown. Transfer to a wire rack to cool. Eat on the day you bake them, ideally while still slightly warm, topped with another dollop of lemon curd.