How to Make It
In a medium saucepan, cover the potato with the water and bring to a boil. Cook until the potato is just beginning to fall apart, about 15 minutes. Drain the potato, reserving 1 cup of the cooking liquid. Pass the potato through a ricer into a large bowl. Add the reserved potato water and let cool slightly. Stir in 6 tablespoons of the flour, the sugar and the yeast. Cover the bowl with a kitchen towel and let stand overnight in a draft-free place.
Uncover the starter; it should be foamy and have a slightly fermented aroma. Add the milk, eggs and 2 1/2 cups of the flour and stir until incorporated. Stir in 1 tablespoon of the butter, 2 tablespoons of the lard and the salt. Add the remaining 3 cups of flour, 1 cup at a time, and stir vigorously to form a silky, slightly soft dough that pulls away from the bowl as you stir, about 8 minutes.
Transfer the dough to a lightly oiled bowl and turn to coat with oil. Cover the bowl with plastic wrap and let the dough rise until doubled in bulk, about 1 1/2 hours.
Gently deflate the dough by turning it in the bowl several times. Cover the bowl and let the dough rise again until doubled, about 40 minutes.
In a small bowl, combine the remaining 6 tablespoons of melted butter and 2 tablespoons of melted lard. Punch down the dough and divide it into 5 equal pieces. Cut 1 piece of dough into 18 equal pieces. Roll each piece into a ball; dip the balls in the melted butter and lard and place 3 balls in each cup of a muffin pan. Continue to form the rolls, working with 1 large piece of dough at a time. Brush the tops of the rolls with the butter and lard and set aside in a draft-free spot until risen just beyond the rims of the muffin cups, about 1 1/2 hours.
Preheat the oven to 425°. Bake the rolls in the lower and middle thirds of the oven for 15 minutes. Remove the rolls from the oven and brush liberally with the butter and lard. Return the rolls to the oven and bake for 5 to 6 minutes longer, or until golden all over. Serve warm or at room temperature