Beat butter at medium speed with an electric mixer in a large mixing bowl until creamy. Gradually add SPLENDA® Sugar Blend, beating well. Add eggs, one at a time, mixing well after each addition. Stir in vanilla.
Melt chocolate in a 1-cup glass measuring cup at HIGH in a microwave oven for 1 to 1 1/2 minutes or until melted, stirring twice. Set aside.
Combine flour, baking powder, and salt in a separate mixing bowl. Gradually add flour mixture to SPLENDA® Sugar Blend mixture, beating until blended. Do not over beat. Divide dough into half. Stir melted chocolate into half of mixture.
Place dough on a lightly floured work surface.
For Checker Board Cookies, shape chocolate dough into 2 rectangular logs approximately 1-inch in diameter. Repeat procedure with vanilla dough. Cut each log lengthwise into quarters. Reassemble logs, alternating chocolate and vanilla to form a checkerboard pattern. Proceed as directed below.
For Pinwheel Cookies, roll chocolate dough into 2 (8- x 9- inch) rectangles. Roll vanilla dough into 2 (8- x 10-inch rectangles) Place the vanilla layer on bottom so that it extends 1 inch beyond the chocolate layer; roll as for a jellyroll. Proceed as directed below.
For Striped Cookies, divide each flavor into 3 balls. Roll each ball into a 7 1/2- x 3-inch rectangle; cut each rectangle into 5 (1 1/2- x 3-inch strips). Stack 5 strips alternating chocolate and vanilla. Proceed as directed below.
Wrap logs in plastic wrap and chill cookie dough for one hour or until slightly firm. (Dough can be frozen up to three months at this point).
Preheat oven to 350 degrees F. Lightly grease cookie sheets.
Remove dough from refrigerator. Slice cookies 1/4-inch thick and place on prepared cookie sheets.
Bake 8 to 10 minutes or until edges of cookies are lightly browned. Cool slightly on cookie sheets; remove to wire racks to cool completely.