Heat oven to 400 F. Combine flour, bacon fat, and salt in a bowl and mix until pea-size crumbs form; add mashed potatoes and milk, mixing to form a smooth dough. Roll out onto a lightly floured surface into a 9½” x 9½” square. Cut into twenty 2½” squares, rerolling dough as needed. Bake on a parchment paper–lined baking sheet until lightly golden, 20 minutes, rotating pans halfway through; transfer to a wire rack to cool.
Heat bacon fat over medium-high heat; cook bread until warmed through and golden, flipping once, about 2 minutes. Serve warm with scallions if you like.