Boston Cream Whoopie Pies

Directions

1. 1

   In a small heavy saucepan, combine the sugar, cornstarch and salt. Stir in milk and cream until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat to low; cook and stir 2 minutes longer.
2. 2

   Remove from the heat. Stir a small amount of hot mixture into egg yolk; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Stir in vanilla. Cool for 15 minutes, stirring occasionally. Transfer to a small bowl. Press waxed paper onto surface of custard. Refrigerate for 2-3 hours.
3. 3

   In a large bowl, cream butter and sugar until light and fluffy. Beat in the egg yolks, egg, vanilla and lemon zest. Combine the cake flour, all-purpose flour, baking soda and salt; gradually add to creamed mixture alternately with buttermilk and mix well.
4. 4

   Drop by rounded teaspoonfuls 2 in. apart onto greased baking sheets. Bake at 400° for 5-7 minutes or until firm to the touch. Remove to wire racks to cool completely.
5. 5

   Spread custard over the bottoms of half of the cookies; top with remaining cookies.
6. 6

   For glaze, place chocolate and butter in a small bowl. In a small saucepan, bring cream just to a boil. Pour over chocolate and butter; whisk until smooth. Stir in confectioners' sugar. Spread over cookies; let dry completely. Store in the refrigerator.