Preheat the oven to 180° C/356° F.
Prick the potatoes with a fork and place on a baking tray. Bake for about 40-50 minutes or until a fork is easily inserted.
For the custard, whisk together the egg yolks, sugar and cornstarch.
Put the milk in a saucepan with the vanilla powder. Bring to a boil, remove from the heat and slowly pour over the egg mixture, mixing continually.
Return the mixture to a low heat and continue stirring until thickened. Then remove from heat and add the butter and vanilla extract.
Cover the surface of the custard with cling film and allow to cool completely.
Once the baked potatoes have cooled enough to handle, scoop out the inside with a spoon. Place the skins back on the tray and bake for another 20 minutes.
Meanwhile, mash the potatoes with the honey, cinnamon, ginger and cardamon (optional). Divide the mash equally between the potato skins and top each one with custard.
Sprinkle generously with caster sugar and either place in the oven under the broiler/grill for about 5-10 minutes or use a blowtorch to caramelize the top.
Serve immediately.