Instructions:
- Preheat oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper cases. Brew coffee and warm milk separately.
- In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
- In a separate bowl, whisk the eggs, vegetable oil, vanilla extract, and buttermilk until just combined.
- Pour the wet mixture into the dry ingredients. Mix on low speed until just incorporated; do not overmix.
- Slowly stream the hot coffee and warm milk mixture into the batter while mixing slowly. The batter will be very thin; this is correct.
- Divide the batter evenly among the 12 liners (about ¾ full). Bake for 18–20 minutes, or until a skewer inserted into the centre comes out with moist crumbs attached.
- Let the cakes cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely before frosting.
- For the frosting, beat the softened butter until light and fluffy using an electric mixer.
- Gradually add the sifted powdered sugar, mixing slowly at first, then increasing speed until the mixture is light and fluffy.
- Gently fold in the marshmallow fluff and vanilla extract by hand using a spatula until just combined, taking care not to deflate the volume.
- Generously cover the cooled cupcakes with the frosting using a piping bag or offset spatula.
- Using a kitchen torch, carefully toast the peaks of the frosting until golden brown and lightly charred. Serve immediately.