Heat butter in medium stockpot. Add onions; cook over medium heat until softened. Add broccoli pieces and chicken stock.
Increase heat to high, cook until mixture comes to a boil. Reduce heat to medium low, simmer until broccoli is tender.
Purée mixture with immersion blender until just small pieces of broccoli remain.
Stir in Cream Sauce Base, salt and pepper. Continue heating over medium low heat, stirring occasionally, until mixture reaches 165ºF.