1.Preheat oven to 350 degrees F (175 degrees C).
2.Lightly grease 2 large baking sheets.
3.Combine water, mashed bananas, honey, vanilla, and egg in a large bowl.
4.Stir in whole-wheat flour and baking powder.
5.Beat dough with an electric mixer on medium speed until ingredients are thoroughly combined, 1 to 2 minutes.
6.Turn dough onto a floured surface and knead until no longer sticky, 5 to 8 minutes.
7.Roll out dough to 1/4-inch thickness and cut into mini shapes with your favorite cookie cutter.
8.Place dough shapes on prepared baking sheets.
9.Bake in preheated oven until cookies are lightly browned, about 20 minutes.
10.Turn off the oven and leave cookies until thoroughly dry and crisp, 30 to 40 more minutes.
11.Remove baking sheets from oven and allow cookies to cool on pans for 10 minutes. Transfer cookies to wire rack to fully cool.