In a large bowl, dissolve the yeast in the warm buttermilk.
Stir in the brown sugar and baking soda.
Set the mixture aside for 5-10 minutes, until it becomes foamy and bubbly.
In a separate bowl, sift together the whole wheat flour, all-purpose flour, baking powder, and salt.
Stir about 1 cup (240 ml) of the dry flour mixture into the activated yeast.
Add the melted butter and beat with a wooden spoon for a minute until combined.
Gradually add the remaining flour, stirring until a shaggy dough forms.
Turn the dough out onto a lightly floured surface.
Knead the dough for 8-10 minutes, adding just enough flour to prevent sticking, until it is smooth and elastic.
Form the dough into a ball.
Place the dough in a lightly buttered bowl, turning it once to coat the top.
Cover with a tea towel and let it rise in a warm, draft-free place for about 1-1. 5 hours, or until doubled in size.
Gently punch down the dough and turn it out onto a lightly floured board.
Roll it into a rectangle approximately 14x7 inches (35x18 cm).
Cut the rectangle crosswise into 15 one-inch-wide strips. To shape, take one strip, twist the ends in opposite directions a few times, and tie it into a simple knot, tucking the ends underneath.
Arrange the knotted rolls a few inches apart on a buttered baking sheet.
Cover lightly with a tea towel and let them rise for another 45 minutes, or until nearly doubled.
Preheat your oven to 425°F / 220°C.
Bake the rolls for 15-20 minutes, or until they are golden brown.
Serve warm.
