Combine 1 cup sugar, syrup, 1 cup milk, and 2 tablespoons butter in a small Dutch oven; cook over low heat, stirring constantly, until sugar dissolves. Continue to cook over medium heat, stirring frequently, until mixture reaches firm ball stage (244°). Remove from heat; add vanilla, stirring well. Place Dutch oven in hot water to keep mixture warm.
Place 1/2 cup sugar in a 10- inch cast-iron skillet; cook over medium heat, stirring constantly, until sugar dissolves and becomes a golden syrup.
Combine remaining 1 1/4 cups sugar, remaining 1/2 cup milk, and soda in a small Dutch oven; bring to a boil, stirring constantly. Add caramelized sugar, stirring well. Cook, stirring frequently, until mixture reaches soft ball stage (234°). Remove from heat, and stir in remaining 1/4 cup butter and sherry; add reserved warm mixture, and beat at medium speed of an electric mixer until thickened.
Pour mixture into a well-buttered 9-inch square baking pan. Cool completely before cutting into 1/2-inch squares; wrap each piece tightly in plastic wrap or waxed paper.