Dough: Combine butter, sugar, cream cheese, egg yolk, lemon juice, extracts and salt. Mix well with an electric mixer on medium speed; beat in flour. Wrap in plastic wrap; chill for at least one hour.
Form dough into one-inch balls. In a small bowl, beat egg white lightly. Dip balls into egg white; roll in pecans. Place 2 inches apart on ungreased baking sheets. Press thumb deeply into center of each cookie. Spoon lemon cheese filling into indents. Bake at 375 degrees for 8 to 10 minutes or until filling is set. Cool slightly; remove to a wire rack. Keep refrigerated in an airtight container.
Lemon Cheese Filling: Beat together all ingredients until smooth.