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1
Preheat oven to 400u00b0F.
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2
Prick sweet potato several times with a fork, place it directly on the oven rack, and bake until very tender, about 1 1/4 hours.
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3
Let cool completely.
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4
Position oven rack in the top 1/3 of oven and preheat oven to 450u00b0F.
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5
Put vinegar into a liguid measuring cup and add the soy milk, this sours the soy milk for a buttermilk replacement.
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6
Peel sweet potato and force it through the medium disk of a food mill, or a large-mesh sieve into a bowl.
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7
Add soured soy milk and whisk until smooth.
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8
In a large bowl, sift together the all-purpose flour, cake flour, baking powder, and sugar (if using dry sweetener, add liquid sweetener to the wet mixture).
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9
Add the shortening and butter and cut them into the dry ingredients using a a pastry blender, until they resemble corn kernals.
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10
Add the wet mixture and stir until a soft, crumbly dough forms.
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11
Turn the dough out onto a well-floured surface and knead 8 to 10 times, until it just holds together.
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12
Roll out and pat the dough into a rectangle 6 by 12 inches.
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13
Using a knife or cookie cutter, cut the dough into 12 rectangular biscuits.
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14
Transfer the biscuits to an ungreased insulated baking sheet (recommended) or doubled regular baking sheets.
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15
Bake until the biscuits have risen and their edges and bottoms are lightly browned, 12 to 14 minutes.
