1. In a large bowl, cream butter and sugar until light and fluffy. Beat in egg yolk and vanilla. Gradually add flour and mix well.
2. Shape into 1-in. balls. Place 2 in. apart on ungreased baking sheets. With a glass dipped in sugar, flatten into 2-in. circles.
3. Bake at 350° for 10-12 minutes or until firm. Remove to wire racks to cool.
4. Combine filling ingredients. Spread on the bottom of half of the cookies; top with remaining cookies.
5. In a microwave, melt chocolate and butter; stir until smooth. Dip each cookie halfway in chocolate; allow excess to drip off. Dip in nuts. Place on waxed paper; let stand until set. Yield: 2 dozen.