Melt butter in a 3-quart saucepan over medium heat. Add sugar and water, stirring until sugar dissolves. Cook over medium heat, without stirring, until mixture reaches soft crack stage (295°).
Remove from heat; quickly add pecans, stirring well. Pour mixture into an aluminum foil lined 15- x 10- x 1-inch jellyroll pan. Let cool.
Melt chocolate morsels in top of a double boiler over simmering water. Spread chocolate over toffee. Cover and refrigerate until completely cooled and hardened. Break into pieces. Store in an airtight container.