Preheat oven to 350° F. Prepare cupcake tin with cupcake liners.
In a medium bowl, whisk together the flour, cinnamon, nutmeg, salt, baking powder and baking soda. Set aside.
In a small saucepan, heat the butter over medium-low heat, whisking constantly. The butter will start smelling sweet and brown specks will appear, remove from heat and continue whisking for 30 more seconds. Pour into stand mixer and let cool. Once the butter is cool, add the buttermilk, egg, sugars and vanilla, and mix until well combined.
In a cup, combine rum and water. Now with the mixer on low and working in batches, alternate adding the flour mixture and the rum mixture into the batter, beginning and ending with the flour. Once both mixtures are added to the batter, continue mixing for another 10 seconds to ensure all the ingredients are combined. Scrape down the sides if necessary.
Fill each cupcake liner about three-quarters full. Bake for 15-18 minutes, until a toothpick inserted in the center of a cupcake comes out clean. Let the cupcakes cool in the tin for 5 minutes and then transfer to a wire rack to cool completely.
Rum Buttercream Frosting
In the bowl of a stand mixer, use a whisk attachment on medium speed to cream the butter until light and fluffy, about 90 seconds. Gradually add the powdered sugar, cinnamon, and vanilla extract. Beat on medium speed until creamy.
Add the rum, starting with one tablespoon and add more until you reach desired consistency (I ended up using 4 tablespoons). Beat on high for 2-3 minutes, until fluffy and whipped.
Butter Rum Syrup
In a sauce pan, melt butter over medium heat. Gently add the rum and heat until warm, about 30 seconds. Add the corn starch and whisk until you reach a thick syrup consistency. Remove from heat and transfer to a bowl to cool.
Assembly
Brush the top of the cupcakes with buttered rum syrup. Transfer the frosting to a frosting bag with desired tip and frost the cupcakes in a swirl motion. Dust with cinnamon if desired. Store in an airtight container at room temperature.
Notes
Decoration: For an added touch, insert cupcakes with a rum-filled alcohol squeeze tube. You will need about 1/2 cup additional rum for this.Store cupcakes in an airtight container at room temperature for 3 days or in the refrigerator for 5 days.Freeze cooled cupcakes wrapped individually and in an airtight container for up to 1 month. Defrost in the refrigerator before bringing to room temperature and adding syrup and frosting.