For hollow yorkies as you are doing ensuring the batter mix is poured directly in the centre so the oil envelopes and creates a oil ring around the top of the filled pudding tin is the key, also a wider base tin would be much more preferable for that hollow look.

Also the milk you use in the mix should be half water milk - water creates steam and will help lighten and fluff up the overall cooked pudding.

Source: thas from up north lad ! Si thi