Whisk buttermilk and egg together in a medium mixing bowl.
Add baking soda and set aside to let the baking soda start to work.
Add the flour blend, guar gum, brown sugar, baking powder, and salt to the bowl of a food processor fitted with a steel blade. Pulse several times to combine. Add butter and process until it’s the size of small peas. (of course, you can also do this using a pastry blender or fork.)
Transfer the flour mixture to a large bowl. Make a well in the center of the flour and add the wet ingredients, stirring just until combined. Make sure to scrape the bottom and sides of the bowl well while stirring.
If using optional add-ins such as currants or caraway seeds, mix those in now
Scrape dough onto parchment-lined baking sheet, and use a rubber spatula to shape dough into a dome. Lightly dust the top of the loaf with flour, and use a sharp knife to slice a very shallow cross shape into the top
Place in center of oven.
Turn the oven down to 375F. Rotate the bread 180 degrees halfway through the baking time. If it browns too quickly, cover it with aluminum foil.
Bake for 30-35 minutes, until the bread has baked through.