Preheat oven to 325°F Line two large baking sheets with parchment paper; set aside. In a large bowl, whisk together sweet potatoes and broth. Add flour, cornmeal, mint and apples and stir until combined. Knead the dough a few times until it holds together. Roll out half of the dough on a generously floured surface until about 1/4-inch thick. Use (2-inch to 4-inch) cookie cutters to cut out treats. Transfer to baking sheets; reroll and cut the scraps. Bake until dried and just slightly golden on the bottoms, 35 to 40 minutes. Turn off the oven, leave oven door ajar and let the treats cool and dry out in the oven for 1 to 2 hours. Store in an airtight container up to 1 week.