As the name implies, corn syrup comes from processing cornflour. Dark corn syrup gets its colouring from the addition of caramel, which also makes it a bit sweeter than light corn syrup. Bakers value cornflour for sweetening because it doesn't crystallise like sugar. Many recipes for candy, jam, fudge and frosting include some form of corn syrup sweetening. Generally corn syrup costs less than sugar. When you do not have dark corn syrup you can use substitutions.
Brown Sugar and Light Corn Syrup
Use brown sugar, water and light corn syrup as a substitute for dark. Brown sugar has a similar taste to the caramel added to dark corn syrup. The proportions for this mixture is 1 cup light corn syrup, 3 to 4 tablespoonfuls of water and 1/4 cup brown sugar. Dissolve the sugar into the water, then blend with the corn syrup. Use in the same measurements as provided for the dark corn syrup.
Light Corn Syrup and Molasses
Mix light corn syrup with molasses and add it to your recipe in the same quantity as dark corn syrup. The proportion is 1/4 cup light molasses to 1 cup light corn syrup.
Make a simple syrup using brown and white sugar with water. Dissolve 1 cup each white and brown sugar completely in one cup warm water. Use in the same quantities as called for in the recipe.
Use maple syrup in place of dark corn syrup. It goes in measure for measure. Just be aware that this might be a little sweeter than the original recipe.
Golden syrup comes from refined cane sugar juice. It's an alternative to corn syrup in baking in equal quantities. It will not have the depth of flavour of dark corn syrup, however.
Molasses comes from processing cane sugar and beet sugar. The best choice as an alternative to dark corn syrup is light molasses so the flavour doesn't overwhelm your recipe. The quantity remains the same.
Replace the dark corn syrup with Karo syrup. These sweeteners are interchangeable. Be aware that this has a more delicate flavour than dark corn syrup.
Add honey to your recipe instead of dark corn syrup. You will find that this turns out sweeter than corn syrup, but provides the same texture. The proportions are the same.