Preheat your oven to 180°C (350°F) for a standard oven or 160°C (320°F) for a fan-forced oven. Line a 12-hole muffin tin with cupcake cases.

In a large bowl, combine the plain flour, cocoa powder, baking soda, baking powder, and both sugars. Stir gently to combine.

In a separate bowl, whisk together melted butter, eggs, vanilla extract, and buttermilk until just combined.

Fold the wet mixture into the dry ingredients gently until just combined.

Spoon batter into prepared cupcake liners, filling 2/3 full. Bake for 18-20 minutes until a toothpick comes out clean. Cool completely on a wire rack.

In a medium saucepan, bring a couple of inches of water to a gentle simmer. In a heatproof bowl, whisk together egg whites, caster sugar, and cream of tartar.

Place the bowl over the saucepan, ensuring it doesn't touch the water. Whisk continuously until it reaches 71°C (160°F), about 5-6 minutes.

Remove from heat and beat mixture with an electric mixer on medium-high speed for 5-6 minutes until fluffy. Add vanilla extract and mix for another 30 seconds.

Transfer frosting into a piping bag and pipe onto cooled cupcakes. Freeze for at least 30 minutes.

Melt dark chocolate in the microwave, stirring every 20 seconds until smooth. Mix in oil until shiny.

Dip each cupcake upside down into the chocolate, allowing excess to drip off. Place right side up on a wire rack. Refrigerate for 30 minutes to set.