In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add granulated sugar, baking powder, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg, milk, and vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Divide dough in half. Knead the cocoa powder into one half of the dough. If necessary, cover and chill dough 30 minutes or until easy to handle.
Preheat oven to 375 degrees F. For cow cookies, on a lightly floured surface, roll the plain dough until 1/8 inch thick. Place 1/2-inch balls of the chocolate dough at 2 inch intervals on top of the plain dough. Roll lightly to flatten the chocolate dough. Use a 3-inch cow-shape cutter to cut dough. Place 1 inch apart on an ungreased cookie sheet. For horses, roll remaining chocolate dough until 1/8 inch thick. Use a 3-inch horse-shape cutter to cut dough. Place 1 inch apart on an ungreased cookie sheet.
Bake in preheated oven for 7 to 8 minutes or until edges are firm and bottoms are very lightly browned. Transfer to a wire rack and let cool.
To add manes and tails onto horses, attach the crumbled shredded wheat biscuits with canned frosting. Makes about 40 cookies.