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1
Melt chocolate in a double boiler or a metal bowl set over a pan of barely simmering water, stirring occasionally, and remove from heat.
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2
Stir in peanut butter.
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3
Beat together butter and sugar with an electric mixer until light and fluffy, then beat in egg and vanilla.
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4
Sift flour, baking powder, and salt into egg mixture and beat until combined well.
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5
Halve dough and roll out each half between sheets of wax paper into a roughly 12- by 8-inch rectangle.
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6
Remove top sheets of wax paper (if they stick too much, chill dough briefly to firm up slightly) and divide chocolate filling between rectangles, spreading it in an even layer.
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7
Tightly roll up each rectangle jelly-roll fashion, beginning with a long side and using wax paper as an aid, to form a 12-inch log.
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8
Wrap rolls in wax paper and then foil.
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9
Chill rolls until firm, at least 2 hours.
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10
Preheat oven to 350F.
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11
Working with 1 roll at a time (keep remaining roll chilled), cut rolls crosswise into 1/8-inch-thick slices and arrange slices 1 inch apart on ungreased baking sheets.
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12
Bake in batches in middle of oven until pale golden and set, 8 to 12 minutes.
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13
Transfer warm cookies to racks to cool.
