I love the taste of this frosting but it was so thin that it just slid of the cake. I didn't add the full amount of heavy cream and added more powdered sugar that it called for. Any other suggestions?

The Best Chocolate Buttercream Frosting

adapted from Delilah Bakery

1 1/2 cup (3 sticks) unsalted butter, softened

1 cup cocoa powder

3/4 teaspoon salt
4 cups powdered sugar
2 teaspoons vanilla
1/4 cup milk
1 cup heavy cream
2/3 cup Ovaltine

Cream together butter, cocoa powder and salt. Butter mixture will be very thick. Turn off the mixer, scrape down the sides of the bowl and add powdered sugar. Turn mixer on low and mix in powdered sugar while adding milk and vanilla extract. As the sugar incorporates, raise the speed of the mixer to beat the frosting. Beat until smooth. In a 2 cup measuring glass, stir together heavy cream and Ovaltine. Turn mixer speed to medium and pour cream mixture into frosting in a slow, steady stream, until youve reached your desired consistency.  You may not need the full amount of Ovaltine and cream.

I've never made this, but reading the recipe, did you beat it long enough? The heavy cream should thicken it I would think...was the cream really cold? That's all I can think of...