Boozy milkshakes and ice cream floats are drawing fans seeking a dessert and cocktail in one—especially for late-afternoon or after-dinner snacks. The trend started at restaurants that specialize in burgers and brews. Soon, towering drinks, lavishly garnished with candies, cake, cookies and other sweets, became Instagram hits. Now, in colder weather, spiked hot chocolate, coffee and other hot beverages double as desserts.