Graham Cracker Fudge

Butter an 8 inch square pan.Melt chocolate in a double boiler over boiling water.Add condensed milk and cook,stirring constantly,until the mixture thickens.Add vanilla,graham cracker crumbs,and 1/2 cup pecans.Mix well.Spread mixture in prepared pan useing knife that has been dipped in hot water (to prevent fudge from sticking to the knife ).Press remaining pecans on top.Cover and chill several hours.Cut into squares.