Orange Marmalade Muffins

I don't remember exactly where I found this recipe. Probably on allrecipes. All I know is that I've had it for years and it's one of our favorite muffin recipes.

2 sticks butter, softened
1 1/4 cups sugar
4 large eggs
2 cups buttermilk
4 3/4 cups flour (add 1/4 cup for high altitude)
2 teaspoons baking soda
2 teaspoons baking powder
1/2 teaspoon salt
2 Tablespoons orange zest
1 cup orange marmalade

Cream together butter and sugar. Add eggs one at a time, mix well. Add buttermilk. Mix in flour, baking soda, baking powder, and salt. Add orange zest and marmalade, stirring until just mixed. Pour into muffin tins and bake at 350 degrees (375 for high altitude)until toothpick comes out clean.