Heat oven to 475℉/245℃.
Line a sheet pan with parchment paper. Set aside.
In a large bowl, whisk together flour, powdered milk, sugar, salt, and baking soda. Set aside.
In a small bowl, whisk together buttermilk and sour cream.
Pour buttermilk mixture over flour mixture.
Mix only to blend.
Turn onto floured surface.
Using floured hands, form into a boule.
Flatten top slightly.
Use a sharp floured knife, to cut top of dough into quarters, stopping about halfway through the dough.
Bake for 8 minutes.
Reduce heat to 375℉/190℃.
Continue baking for 15-20 minutes, until done.