To prepare the cookies, whisk together the flour, protein powder, baking powder, and salt in a medium bowl. In a separate bowl, whisk together the butter, egg, vanilla extract. Stir in the honey and vanilla stevia. Add in the flour mixture, stirring just until incorporated. Transfer the dough to the center of a large sheet of plastic wrap, and shape into a 1″ rectangle. Cover the top with another large sheet of plastic wrap. Chill the dough for at least 1 hour.
Preheat the oven to 350°F, and line two baking sheets with parchment paper.
Leaving the cookie dough between the sheets of plastic wrap, roll it out until desired thickness. Lightly flour your cookie cutter, and press it into the dough. Peel the unused dough away from the shapes, and place them onto the prepared baking sheets. Reroll the unused dough, and repeat.
Bake the cut out cookie dough at 350°F for 8-10 minutes. Cool on the baking sheet for 5 minutes before turning out onto a wire rack to cool completely.
To prepare the icing, stir together the Protein Powder and milk in a small bowl. Spoon into a zip-topped bag, and snip off the corner. Pipe onto the cooled cookies.