5 cups granulated sugar, divided
2 cups half-and-half
1/4 cup butter
1/2 cup milk
2 cups miniature marshmallows
1 teaspoon vanilla extract
1 cup chopped pecans
Combine 4 cups sugar, half-and-half and butter in a heavy Dutch oven. Cook over low heat, stirring gently, until sugar dissolves. Cover and cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan.
Sprinkle remaining 1 cup sugar in a large heavy skillet; cook over medium heat, stirring constantly, until sugar melts and turns light golden brown. Pour caramelized sugar and milk into fudge mixture. (Mixture will lump but will become smooth with further cooking.) Cook over low heat, stirring constantly, until caramelized sugar dissolves. Continue cooking, without stirring, until mixture reaches soft ball stage (240 degrees F). Cool to 160 degrees F.
Add marshmallows and vanilla extract. Beat with a spoon until marshmallows melt and mixture begins to thicken. Stir in pecans. Pour into a buttered 9-inch square pan, spreading with a spatula. Cool candy, and cut into squares.