In a medium saucepan over low heat, melt the butter. Once melted, add the semi-sweet chocolate and stir continuously until completely smooth. Remove from heat immediately to prevent scorching.

Add the cocoa powder and salt to the melted chocolate mixture. Stir until a thick paste forms. This step 'blooms' the cocoa, intensifying its flavor.

Slowly whisk in the heavy cream and vanilla extract until the mixture is uniform and glossy. Transfer the mixture to a heatproof bowl. Press a piece of plastic wrap directly onto the surface to prevent a skin from forming and refrigerate for at least 2 hours, or until completely chilled and firm.

Once chilled, transfer the chocolate mixture to the bowl of a stand mixer fitted with the whisk attachment (or use a hand mixer). Start on low speed and gradually increase to medium-high. Whip for 3-5 minutes, or until the filling is light, fluffy, and has stiff peaks.