Preheat oven to 375 degrees.
In a large mixing bowl, whisk together the flour, baking soda, baking powder, salt, and sugar. Whisk in add-ins, if desired.
In a medium mixing bowl, whisk together the butter, sour cream, buttermilk, and eggs.
Pour the wet ingredients into the dry ingredients, using a whisk, large fork, or your hands, mix just until combined. Take care not to overmix. The dough should be slightly tacky but with small visible streaks of flour. If the dough is too tacky, add 1 tablespoon of flour at a time until the correct consistency has been reached.
Turn the dough onto a very lightly floured surface. Gently knead a few times to form a round, shaggy loaf. Transfer it to a piece of parchment paper.
Using a sharp knife, score a deep X into the top of the loaf. It should be about 1-inch in depth.
Brush with topping, add extra sugar if desired. Place in a large cast iron skillet or dutch oven, or set on a cookie sheet.
Place in the oven and bake for 45-50 minutes or until the center of the loaf is fully baked.
Remove from the oven and let cool for 10 minutes before transferring to a wire rack to cool for an additional 10 minutes. Cut and serve within 3 days.