Melt chocolate by combining semi-sweet chocolate chips and unsalted butter in a microwave-safe bowl. Heat at 50% power for 30 seconds intervals, stirring until smooth.

In a chilled mixing bowl, whip 1 cup of heavy whipping cream on medium-high speed until soft peaks form, approximately 2-3 minutes.

Gently fold half of the whipped cream into the cooled chocolate mixture using a spatula, maintaining airiness.

Add powdered sugar to the remaining heavy whipping cream and whip until stiff peaks form, about 1-2 minutes.

Carefully fold half of the sweetened whipped cream into the chocolate mixture, keeping it fluffy.

Fill serving cups with the mousse mixture, leaving some space at the top for presentation.

Cover the cups with plastic wrap and refrigerate for at least 4 hours or until set.

Before serving, top each cup with a dollop of remaining whipped cream and sprinkle with cocoa powder or berries.