To make the cupcakes:

1. Preheat the oven to 350 degrees. Line a muffin tin with paper cupcake liners. 
2. In a mixer using the paddle attachment, cream the sugar, coconut oil, and miso paste together until smooth and creamy. Add in the egg and vanilla extract and mix until full incorporated.
3. In a large mixing bowl, whisk together the flour, cocoa powder, baking powder, and baking soda. Slowly add to the wet mixture in batches and blend until smooth.
4. With the mixer on, add the buttermilk and boiling water to the batter until fully incorporated (scraping down the sides of the bowl).
5. Fill the cupcake liners with the batter about 3/4 full. Bake for 20-25 minutes, or until a toothpick inserted into the middle comes out clean.
6. Remove from the oven and let cool completely before frosting the cupcakes.

To make the frosting:

1. In a mixer using the paddle attachment, beat the softened butter at medium speed until smooth and creamy.
2. Slowly add in the confectioners sugar in batches until fully incorporated, scraping down the sides.
3. Add in the vanilla extract and salt and beat at low speed until combined. Raise speed to medium and beat until smooth and creamy.
4. With the mixer on, add in the milk and mix another 2 minutes until desired consistency.
5. Frost the miso cupcakes with a piping bag or a frosting knife. Top with your choice of sprinkles!