Personally I wouldn't go cake if you want low syn, unless you make or buy some cupcakes (with a proper recipe but use flora light or I can't believe its not butter light) me and my mum always used those when baking even before slimming world and our cupcakes actually work out at around 8 - 10 syns each (can't remember exactly how much) and they're lovely. Also if you buy shop cupcakes you can check syns and most are under 15 syns as long as they're not massive muffins ect.
I don't like low syn cake recipes, sweetener just doesn't bake properly.
There are loads of yummy looking tart recipes or meringue based things if you want a low syn pudding though.