1 cup dark brown sugar, firmly packed
1/2 cup evaporated milk
2 1/2 tablespoons (1.25 oz/35g) butter or stick margarine
2 teaspoons light-colored corn syrup
Dash of salt
2 cups confectioners' sugar
2 1/2 teaspoons vanilla extract
Combine brown sugar and next 4 ingredients (brown sugar through salt) in a medium saucepan, and bring to a boil over medium-high heat, stirring constantly.
Reduce heat, and simmer until thick (about 5 minutes), stirring occasionally. Remove from heat.
Add confectioners' sugar and 2 1/2 teaspoons vanilla; beat at medium speed of a mixer until smooth and slightly warm. Cool 2 to 3 minutes (frosting will be thin but thickens as it cools).
Place 1 cake layer on a plate; spoon 1/2 cup frosting on to cake layer spreading to cover. Top with remaining cake layer.
Frost top and sides of cake, and garnish top of cake with pecan halves (optional).
Store cake loosely covered in refrigerator.