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1
Heat the oven to 425 degrees F and arrange a rack in the middle.
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2
Using a pastry brush, coat a 9-inch round cake pan with half of the melted butter.
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3
Set the pan and the remaining melted butter aside.
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4
Place the cornstarch, tapioca flour, sorghum flour, baking powder, guar gum, and salt in a large bowl and whisk to combine and break up any lumps.
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5
Using a pastry blender or your fingers, cut the cold butter and vegetable shortening into the dry ingredients until they are pea-size pieces.
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6
Add the buttermilk and stir with a rubber spatula just until the dough forms a ball.
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7
Turn the dough onto a work surface dusted with tapioca flour and knead just until smooth, about 3 to 4 turns (do not overwork the dough).
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8
Pat the dough into a 1-inch-thick disk about 6 inches in diameter.
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9
Using a 2-inch round cutter dipped in tapioca flour to prevent sticking, cut out as many biscuits as possible.
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10
Form the scraps into another 1-inch-thick disk and continue to cut biscuits until you have 8 total.
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11
Discard the remaining scraps.
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12
Arrange the biscuits in a circle in the prepared pan about 1/4 inch apart (the center of the pan will be empty).
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13
Brush the tops of the biscuits with all of the remaining melted butter.
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14
Bake until golden brown, about 20 to 25 minutes.
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15
Remove from the oven and let cool slightly in the pan on a wire rack.
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16
Serve warm with butter, agave syrup, or preserves.
