Add the condensed milk, brown sugar, maple syrup, and butter to a saucepan set over medium-high heat. Bring to a boil; reduce heat to low and cook until it resembles a thick caramel sauce, about 7 minutes. Test for doneness by dropping a small amount of the maple cream into a glass of cold water – it should solidify into a soft, pliable ball. Chill thoroughly, until the fudge solidifies/hardens.