Preheat the oven to 180C/350F and line a 12 hole cupcake tray with liners.
Place the butter and sugars into a large bowl, or the bowl of your stand mixer and beat until light and fluffy, around 2 minutes on medium-high speed.
Add in the vanilla and eggs, scraping down the sides as necessary. Mix on medium speed until well incorporated and smooth.
Add in the flour, baking powder, salt and spices. Mix on low until the mixture starts to come together, then add the black treacle. Mix until just incorporated, then add the eggnog. Now turn the speed up to high and beat until everything is smooth and well combined.
Divide the batter between your liners, I placed 3tbsp of batter in each cupcake case. This batter doesn't rise as much as my normal cupcakes, it's not meant to, the texture is still perfect, it just means you can put a bit more batter in your liners without fear of it overflowing!
Place in the oven for 18-22 mins, until risen and an inserted skewer into the centre comes out clean. You won't really be able to tell if they're golden because of the colour of the batter.
Transfer to a wire rack as soon as possible to cool completely.
Once cool, make the frosting. Place butter, icing sugar, salt and nutmeg into a large bowl, or the bowl of your stand mixer and mix on low until the sugar starts to mix with the butter. This will avoid an icing sugar dust cloud! Once there are no lumps of icing sugar, add in 1tbsp eggnog and mix on medium-high speed for 30 seconds. Add in the 2nd tbsp of eggnog and repeat.
At this stage you will need to pick up a bit of the frosting on a spoon/spatula and check the consistency. You want it to stay on the spoon/spatula but creamy enough to pipe. If your mixture is too stiff, add in the final tbsp of eggnog. I needed all 3tbsp of eggnog in my frosting.
Continue to beat, turning the mixer to high and mix for 3-4 minutes, or until the mixture pales and takes on a whipped, light as air consistency.
Attach an open star nozzle to a piping bag. Half fill the piping bag, then pipe a swirl onto each cupcake. You can also spoon on frosting, you probably won't use all of the frosting if you spoon it on.
Sprinkle each cupcake with a little nutmeg and some festive sprinkles, if you would like to.
Eggnog Gingerbread Cupcakes will keep in an airtight container, in the fridge, for 5 days. Best served at room temperature.