A moist, perfumy spice cake sweetened with dark sugarcane syrup, gateau sirop is a dessert and coffee an’ staple in cafes and fine dining rooms in Louisiana Cajun country, where cane sugar is harvested. Whipped cream is a welcome crown; at the venerable Prejean’s of Lafayette, it comes gilded with Frangelico cream sauce; at Cafe des Amis in Breaux Bridge, it is dotted with roasted pecans and available a la mode.