Combine all the dry ingredients in a bowl. Work in the butter/margarine, egg yolk and vanilla extract. Refrigerate the dough for 1 hour, then press the dough into greased heart-shaped tins.
Bake at the centre of the oven for 6 minutes at 200°C.
While the hearts are baking, mix together the ingredients for the filling.
Divide the filling between the pre-baked heart tins.
Bake for a further 5-7 minutes.
Allow the hearts to cool before removing them from the cases.
Dust with Siúcra/McKinney's Icing Sugar to serve.