In a medium microwave-safe bowl, combine the semi-sweet chocolate chips and heavy whipping cream. Microwave in 30-second intervals, stirring well after each one, until the chocolate is completely melted and the mixture is smooth.

Add the softened unsalted butter and unsweetened cocoa powder to the warm chocolate mixture. Stir until the butter is fully melted and the cocoa powder is completely incorporated. The mixture will be thick and glossy.

Cover the bowl with plastic wrap and refrigerate for at least 1-2 hours, or until the mixture is firm to the touch.

Transfer the chilled mixture to the bowl of a stand mixer fitted with the whisk attachment (or use a hand mixer). Beat on medium-high speed for 2-4 minutes, or until the filling becomes light in color, fluffy, and holds its shape.