Mix the cornstarch with 4 tablespoons of cold water until dissolved. Set aside.

In a saucepan, combine lemon juice, orange juice, 1 cup of water, and sugar.

Stir over medium heat until the sugar dissolves.

Increase heat and bring to a boil.

Remove the saucepan from the heat.

Stir in the cornstarch mixture.

Return the saucepan to medium heat.

Stir constantly until the sauce thickens.

Remove the pan from heat.

Add butter and stir until melted and fully incorporated.

Serve immediately.