Melt the Chocolate: Place the chopped dark chocolate in a microwave-safe bowl. Heat in 30-second bursts, stirring in between, until it is completely melted and smooth. Set it aside to cool for about 10-15 minutes. It should be cool to the touch but still liquid.

Beat the Butter: In the bowl of a stand mixer fitted with the paddle attachment (or using a large bowl and an electric hand mixer), beat the softened butter on medium-high speed for a full 5 minutes. The butter should become very pale, light, and fluffy. This step is crucial for the final texture.

Sift Dry Ingredients: While the butter is beating, sift the icing sugar and cocoa powder together into a separate bowl. This ensures there are no lumps in your finished buttercream.

Combine Butter and Sugar: With the mixer on low speed, gradually add the sifted sugar and cocoa mixture to the whipped butter, about one cup at a time. Mix until just combined after each addition. I find that scraping down the sides and bottom of the bowl with a spatula after every two additions prevents any pockets of unmixed ingredients.

Add Liquids: Once all the sugar mixture is incorporated, the frosting will be very thick. Pour in the cooled melted chocolate, vanilla extract, and salt. Mix on low speed until just combined.

Whip to Perfection: Increase the mixer speed to medium-high. While it's running, slowly drizzle in the double cream or milk. Continue to beat for another 3-5 minutes until the buttercream is gloriously smooth, airy, and has increased in volume. It should be a perfect consistency for spreading or piping.