Instructions: For the milk chocolate layer, line an 8-inch square pan with plastic wrap, leaving an overhang. Melt milk chocolate in the top of a double boiler above gently simmering water and whisk until smooth.

Beat the peanut butter and oil using an electric mixer until smooth. Mix in the milk chocolate and cookies until well blended. Pour into the prepared pan, smoothing the top with a spatula. Bang the pan on a work surface to even contents. Cover with plastic wrap and refrigerate until firm, for about 2 hours.

For the bittersweet chocolate layer, melt the bittersweet chocolate in the top of a double boiler above gently simmering water and whisk until smooth. Cool to tepid.

Meanwhile, whip cream to soft peaks. Quickly fold 1/2 of whipped cream into the chocolate, then fold in the remainder. Pour over the firm, chilled milk chocolate layer. Spread with spatula until smooth. Bang on a work surface to eliminate air bubbles. Cover with plastic wrap and refrigerate until firm, for about 4 to 6 hours. (This can be prepared several days ahead.)

To serve, remove from the refrigerator at least 1 hour before serving. Immediately lift the chocolate dessert out of the pan. Cut lengthwise into 1-inch wide strips using a knife rinsed in hot water and dried, then cut each strip in half crosswise to form two 4- by 1-inch bars. Sieve cocoa powder over the tops of the bars.

Place 2 bars on 8 plates, resting 1 edge of 1 bar dramatically atop the second bar. Let sit at room temperature about 1 hour before serving.

This recipe yields 8 servings.

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