Preheat oven to 300 degrees.
Mix all ingredients, except extras, in a mixing bowl until well combined.
Add in extras like fresh fruit or ground, dried fruit. Be sure to check that it's something safe for dogs to ingest.
On a floured surface, roll your dough out to 1/2-1/4 inch thick. Dough will be wet. Feel free to add additional flour, one tablespoon at a time, until it's a consistency you feel comfortable working with.
Cut into desired shapes using cookie cutters, or just cut into squares using a sharp knife.
Place on the cookie sheet so they're just barely not touching. You can use parchment or silicone baking mat, though I've found it unnecessary. This recipe makes more than I can tightly fit on one large cookie sheet, so you may need to do two rounds of baking.
Bake until firm and golden brown, about 30 minutes.
Cool on a wire rack before serving.
Store in an airtight container, preferably in the fridge for the longest shelf-life.