Chop chocolates and transfer into a heat proof bowl. Heat the cream in a small saucepan until bubbles form around the edge of the pan. Pour cream over the chocolate. Let sit for 1 minute, and then stir using a rubber spatula until combined. Add the butter to the chocolate, stirring until combined.
Sift the powdered sugar and whisk together with salt, milk and vanilla in another bowl until combined. Pour the sugar mixture onto the chocolate mixture, then stir until combined and smooth.Let sit at room temperature until thickened. Beat with an electric mixer until fluffy. Pipe or spread onto cupcakes.