Mix crushed digestive biscuits with melted butter in a bowl; press into the bottom of a 7-inch cake pan. Refrigerate until firm, about 30 minutes.

Dissolve cocoa powder in hot water in a small bowl.

Pour half of the heavy cream into a bowl; whisk with an electric mixer until soft peaks form. Fold in cocoa powder mixture and melted chocolate until well blended.

Beat half of the cream cheese and 2 1/2 teaspoons sugar together in a bowl. Fold in chocolate mixture until thoroughly combined. Spread over digestive biscuit crust. Chill until set, 2 to 3 hours.

Combine remaining cream cheese, remaining sugar, and Irish cream liqueur in a bowl; beat until blended.

Whisk remaining heavy cream in a separate bowl with an electric mixer until soft peaks form. Fold into the cream cheese mixture. Spread gently over chocolate layer in the pan.

Chill until set, 2 to 3 hours.