In a large bowl, cream shortening and sugars. Add eggs, one at a time, beating well after each. Beat in chocolate and vanilla. Combine buttermilk and water. Combine the cake flour, baking soda and salt; add to creamed mixture alternately with buttermilk mixture.
Pour into a greased and floured 13-in. x 9-in. baking pan. Bake at 350° for 30-35 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack.
For filling, in a small saucepan, combine brown sugar and all-purpose flour. Stir in milk until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer.
Remove from the heat. Gently stir in butter and vanilla. Cool to room temperature without stirring. Spread over cake to within 1/2 in. of edges; sprinkle with pecans. Refrigerate for 30 minutes or until set.
For frosting, in a heavy saucepan over low heat, combine the sugar, egg whites, water, corn syrup and cream of tartar. With a portable mixer, beat on low speed for 1 minute. Continue beating on low over low heat for 8-10 minutes or until frosting reaches 160°.
Pour into the large bowl of a heavy-duty stand mixer; add marshmallows. Beat on high for 7-9 minutes or until stiff peaks form. Carefully spread over cake. Pipe thin lines of melted chocolate over cake; gently pull a toothpick or sharp knife through lines in alternating directions. Store in the refrigerator. Yield: 15 servings.