In a large mixing bowl beat the butter with an electric mixer on medium to high speed about 30 seconds or until softened. Add the brown sugar and beat until fluffy. Add the egg and vanilla. Beat until thoroughly combined, scraping the sides of the bowl occasionally. Then beat in the flour until well mixed.
Shape the dough into 3/4-inch balls. Place about 2 inches apart onto ungreased cookie sheets. Tuck 3 or 4 small pecan halves under each ball. Slightly flatten balls.
Bake in a 375 degree F oven for 9 to 10 minutes or until lightly browned on the bottoms. Remove cookies and cool on wire racks.
In a small heavy saucepan melt the chocolate pieces and shortening over low heat, stirring occasionally. Using a knife, swirl chocolate on the tops of the cookies. Makes about 42 cookies.