Why This Works
This drink is an emulsion in a mug. The butter carries the spices — cinnamon, clove, nutmeg, and allspice are all fat-soluble, so they bloom in the batter in a way they never would stirred into water. When the hot water hits, the butter disperses and coats every sip with spice.
Dark brown sugar matters: its molasses content echoes the rum alternative's own molasses base, so the sweetness and the spirit taste related. This is a drink about richness and warmth rather than balance in the citrus sense — the pinch of salt is what keeps all that roundness from going flat.
The Alcohol-Free Swap
Alcohol-free rum was practically made for this drink — its molasses backbone is the through-line connecting the brown sugar, the spices, and the butter.