Heat the buttermilk to lukewarm, remove from heat, and stir in yeast to dissolve. Meanwhile, sift flour with baking soda, salt, and sugar. Cut the vegetable shortening and butter into the flour mixture until it resembles coarse meal. Refrigerate.
When buttermilk and yeast are at room temperature, beat the liquid into the flour mixture with 10 to 20 quick strokes. Refrigerate again for 10 minutes.
Sprinkle a counter or pastry marble with flour, turn the dough onto it, and knead 15 to 20 strokes, until the dough just coalesces. Add flour when kneading if necessary (especially in humid weather). Pat out to a