In a large bowl, cream butter and sugars until light and fluffy. Beat in egg and vanilla. Combine flour and salt; gradually add to the creamed mixture and mix well.
Divide dough in half. Add cocoa to one half and mix well. Roll each half between waxed paper into a 9-in. square. Cut both squares into three 3-in. strips. Cut the strips in half lengthwise to make six 4-1/2x3-in. rectangles.
Place one cream-colored rectangle on a large piece of plastic wrap; top with a chocolate rectangle. Repeat layers twice. Wrap in plastic and refrigerate for 2 hours. Unwrap and cut widthwise into eighteen 1/4-in. slices.
Cut each slice with a horse-shaped cookie cutter dipped in flour. Place 1 in. apart on ungreased baking sheets. Bake at 375° for 8-10 minutes or until edges are lightly golden. Let stand for 2 minutes before removing to wire racks to cool.