Preheat the oven to 190 °C / 375 °F (170 °C fan) and line a cupcake or muffin tin with 12 food-safe plant pot cases, or just regular cupcake liners.
Gently melt the chocolate by using a microwave or placing it in a heatproof bowl over a pot of simmering water. When melting chocolate in the microwave, it's crucial to stir it every 20 seconds to prevent burning. Set aside.
Cream the butter using a stand or hand-held mixer, until it is pale and creamy, and has no lumps. Add the sugar and mix well.
Incorporate the eggs individually, ensuring thorough mixing after each addition. Next, blend in the melted chocolate and yogurt until the mixture is smooth.
Gradually add the flour, cocoa powder, baking powder, bicarbonate of soda, and salt, stirring gently to combine without over-mixing. Finally, pour in the milk and vanilla extract, mixing gently until the batter is fully combined.
Fill the cupcake cases ¾ full, as the cupcakes will rise when baking. Bake in the oven for 20–22 minutes, or until a skewer or toothpick inserted into the centre comes out clean. Let it cool completely on a wire rack.