Preheat the oven to 350ºF and prepare a cupcake pan with 6 liners.
In a small bowl, combine the flour, baking powder, ginger, cinnamon, nutmeg, allspice, and salt and mix together well. Set aside until use.
In the bowl of a stand mixer with the paddle attachment, cream the butter and brown sugar together until fully combined and fluffy.
Add in the egg and beat until combined, then add the molasses, rum, and vanilla, beating on low to medium speed until fully combined.
Add in the dry ingredients and mix until just combined, then add in the milk and mix until the batter is combined and smooth.
Divide the batter between the 6 paper liners (using a cookie scoop for even portions) and bake for about 18 minutes, or until a toothpick comes out clean.