Grease a deep 12-hole muffin tin with butter.
Mix candied peel, sultanas, and cranberries with spiced rum in a bowl.
Combine flour, sugar, and salt in a mixer bowl.
Whisk yeast with some of the water until dissolved, then add to flour with remaining water and whole eggs.
Beat the mixture until a dough forms, then incorporate butter gradually and add orange zest.
Beat for 5 minutes until smooth and let prove covered for 1 hour until doubled in size.
Knock back the dough and knead in the soaked dried fruit.
Divide dough into 12 balls, place in tin, and prove for 45 minutes.
Preheat oven to 190°C.
Brush with egg wash and bake for 20 minutes until golden.
Whisk double cream until it separates into solid butter and liquid buttermilk.
Strain through a muslin cloth, squeeze out liquid, and discard the liquid.
Beat the solid butter with brown sugar and rum until fluffy.
Serve the warm panettones with the chilled rum butter.