Butter the inside of a 1.2 litre (2 pint) straight-sided heatproof dish with a little of the butter, checking first that it will fit into the slow cooker pot.
Slice the croissants thickly and spread one side of each slice with the remaining butter. Mix together the sugar and spice. Arrange the croissants in layers in the dish, sprinkling each layer with the spiced sugar and the pecans.
Whisk the milk, whole eggs, egg yolks and vanilla extract in a bowl. Pour into the dish and leave to soak for 15 minutes. Preheat the slow cooker if necessary; see the manufacturer's instructions.
Cover the top of the dish loosely with buttered foil and lower it into the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the dish, cover with the lid and cook on low for 4–4½ hours or until the custard is set and the pudding well risen. Lift the dish out of the slow cooker pot using a tea towel. Dust with sifted icing sugar. Scoop into bowls and serve with cream.