Homemade Vegan Buttermilk: In a large measuring cup, stir together 1 ½ cups soy milk (or another non dairy milk) and 4 teaspoons white vinegar (apple cider vinegar or lemon juice). Set aside to curdle.
Preheat the oven to 400ºF. Grease a 9" or 10" cast iron skillet (or similar) with softened vegan butter or use a slice of parchment paper. Melt the butter.
Dry Ingredients: Whisk together the flour*, baking soda, baking powder, salt, and sugar. *Measure 4 cups of flour using a kitchen scale or the spoon and level method (see notes). Remove ½ a cup of flour and set aside.
Wet Ingredients: Make a well in the flour mixture add in the vegan buttermilk, plant based yogurt, melted butter, and chopped raisins (currants). Stir with a wooden spoon until just combined. The dough will be sticky- add more of the reserved flour as needed to prevent the dough from sticking.
Shaping the loaf: Turn the dough out onto a floured work surface. Add a sprinkle of flour over the sticky dough ball (from the reserved flour) as needed. With lightly floured hands, gently shape the dough into a round loaf (~6-7"). Flatten the rounded dough slightly. Do not overwork the dough or the bread will be dense instead of tender.
Score the bread: Transfer the round to the prepared cast iron pan. Using a sharp knife, score a large 'X' (~½" deep) in the top of the dough ball (see photos). This allows the center to bake.
Bake the bread: Place the bread in the preheated 400º F oven and bake for 40-55 minutes or until lightly golden and bottom sounds hollow when tapped. Internal temperature 195º F (90º C). Loosely tent the bread with aluminum foil if its taking on too much color.
Let cool: Remove from the oven, transfer to a wire rack, and let cool for ~15 minutes before slicing.