Preheat your oven to 350°F. Line a standard 12-cup muffin tin with paper liners and set them aside.

Make the cupcake batter. Sift flour, baking powder, baking soda and salt in a medium bowl and whisk to blend evenly.

Combine soft butter with sugar, oil and vanilla extract in a large bowl and beat using an electric hand mixer on medium-high speed for about 2 minutes until very pale, creamy and fluffy. Add egg and beat until well incorporated and fluffy, about 30 seconds on medium-high speed. Mix in sour cream.

Add one third of the flour mixture to the butter mixture and beat on medium-low speed until mostly incorporated. Add half of the milk and beat until well blended, about10-15 seconds. Beat in half of the remaining flour mixture followed by the last of the milk on medium speed just until the batter is smooth, but do not over beat. Finally, mix in the rest of the flour mixture on low until evenly combined. Then, just when the flour is combined, beat on medium-high speed for 5-10 seconds to emulsify the batter.

Divide the batter evenly among paper cups, filling them about two-thirds full and bake for 18-20 minutes until just lightly golden and a toothpick inserted into the center comes out clean. Transfer them to a wire rack to cool completely.

Make the buttercream. First combine cocoa powder and boiling water in a small bowl and whisk until it forms a smooth paste. Let it cool completely.

Combine soft butter and icing sugar in a large bowl and beat with an electric hand mixer on medium-high speed for about 2 minutes until very pale, creamy and whipped. Add the cooled cocoa powder mixture, vanilla and salt and beat until very pale and fluffy. Add 1 tablespoon of cream and beat until smooth. Add another tablespoon and beat again. Add half of the melted cooled chocolate and mix until just incorporated. Add another 2 tablespoons of cream (one tablespoon at a time) and beat between each addition. Add the last of the melted chocolate and mix it in, then finally beat in the last tablespoon of cream. This gradually addition of cream ensures that the frosting doesn’t split and gives you a smooth creamy consistency.

Use immediately to top the cupcakes. This frosting pipes very nicely. It will firm up as it cools further and the chocolate sets. Decorate with your favorite sprinkles! This frosting is very stable so store at room temperature for up to 4 days. Do not refrigerate.