Preheat the oven to 160°C / 325°F / Gas Mark 3.
Grease a 1 litre ovenproof dish with a little butter.
Spread the hot cross bun slices with butter on both the internal sides.
Arrange the buns in the ovenproof dish with the buttered sides together and the cross sides face up.
In a small pan, heat the cream and milk together until just warm.
In a small bowl, whisk together the eggs and egg yolk.
Temper the eggs by adding a bit of the heated cream and milk and whisking rapidly. Add a little more, whisk and then add the remainder, whisking all the while. (This avoids scrambled eggs!)
Add the yogurt and whisk together.
Pour the mixture over the buns a little at a time, making sure it has time to soak in. (I gently lifted the tops of the hot cross buns to make sure the bottoms were thoroughly soaked, then replaced the lids and continued pouring the custard mixture over the buns.)
Sprinkle the pudding with the demerera sugar.
Place the ovenproof dish on a baking sheet and bake for about 30 minutes or until the pudding is puffed up, set and golden brown.
Serve with custard or cream.