1. Place 2/3 cup chips in a small bowl and microwave on high 1 minute, until chips melt; stir until smooth. Set aside to cool.
2. Whisk together flour, baking powder, and salt. In a large bowl, with an electric mixer on medium, beat butter and sugar until fluffy, about 2 minutes. Beat in egg, vanilla, and cooled chocolate. On low, beat in flour mixture until combined. Form dough into a rectangle measuring 12 x 2-1/4 x 3/4 inches. Wrap in plastic and chill until very firm, about 4 hours.
3. Heat oven to 350 degrees F. Grease baking sheets. Cut dough into 1/4-inch slices. Arrange 1 inch apart on baking sheets; bake 9 to 10 minutes, until set. Cool 5 minutes on sheets and transfer to wire racks. Repeat with remaining dough.
4. Melt remaining chocolate chips. Dip each cookie upside down into chocolate; smooth surface with a small, angled spatula. When you have dipped 12 cookies, mark the middle of each one by dragging a knife across the center. Place candies on cookies in domino patterns. Repeat with remaining cookies.