This is a recipe for a basic butter sauce which you can adapt to many variations. 
You might add fresh herbs, minced ginger, roasted peppers or tequila and lime for example.  This sauce is great with fish or sea food.

Ingredients:
2 Tablespoon minced shallot ( regular onion will also work )
1/4 cup lemon juice
2 Tablespoons cream
8 ounces cold cubed unsalted butter
Salt

Place shallot and lemon juice in a heavy sauce pan and reduce by 1/2 over medium low heat.  Add the cream and reduce by 1/2 again. This mixture should be slightly thick.  Turn the heat down to low. Start to add the butter one cube at a time. Whisk in each cube until it's melted. Once one is just about melted add the next one and so on until all the butter is incorporated. Serve immediately or hold at room temp until needed.