Melt the chocolate first using either the microwave or like I do: I put my chocolate in a glass measuring cup, place that in a pot with water in it (making sure the water isn’t so deep it flows into the measuring cup) and heat on the stove over medium, stirring occasionally until fully melted. Allow to cool to room temp.
Once the chocolate is cooled, cream together butter and shortening until smooth. Slowly add 2 tsp. vanilla and 5 tbs. of cream, then add the chocolate. Gradually add the powdered sugar and scrape down the bowl very well once it’s all added. Slowly add more cream and corn syrup a tbs at a time until frosting reaches desired consistency (you may not need a lot, depending on how hot your kitchen is and how humid it is outside).