In the bowl of a stand mixer, whisk together 1 1/4 cups (156 grams) flour, sugar, salt, yeast, and baking soda by hand.
In a medium saucepan, heat buttermilk, water, and butter over medium-low heat, stirring occasionally, until an instant-read thermometer registers 120°F (49°C) to 130°F (54°C). Add warm buttermilk mixture to flour mixture; using the paddle attachment, beat at medium-low speed until combined. Reduce mixer speed to low, and gradually add 2 1/2 cups (313 grams) flour, beating just until combined and stopping to scrape sides of bowl.
Switch to the dough hook attachment. Beat at low speed until a soft, somewhat sticky dough forms, 10 to 12 minutes, stopping to scrape sides of bowl and dough hook; add up to remaining 1/3 cup (42 grams) flour, 1 tablespoon (8 grams) at a time, if dough is too sticky. Turn out onto a lightly floured surface, and shape into a smooth round.
Lightly oil a large bowl. Place dough in bowl, turning to grease top. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size, 30 to 45 minutes.