Make chocolate layers: Place baking sheet, bottom side up, on work surface. Cover bottom side with foil, tucking ends under. Mark 15 x 9-inch rectangle on foil. Place chocolate in top of double boiler set over barely simmering water (do not allow bottom of pan to touch water); stir until melted and smooth. Pour onto foil; spread to cover rectangle. Chill until just set but not hard, about 8 minutes.
Cut chocolate rectangle lengthwise into three 3-inch-wide strips. Cut strips crosswise into six 2 1/2-inch-long pieces. Chill 1 hour. (Can be made 1 week ahead. Cover; keep refrigerated.)
Make mascarpone cream: Using electric mixer, beat first 4 ingredients in medium bowl to blend. Add 1/2 cup cream; beat until soft peaks form. Add 1/2 cup cream; beat until thick and stiff. Cover; chill at least 1 hour and up to 3 hours.
Place 1 chocolate piece on each of 6 plates. Spread 3 tablespoons mascarpone over each. Top with another chocolate piece; spread mascarpone over. Top with third chocolate piece. (Can be made 6 hours ahead. Cover; chill.) Dust with powdered sugar. Spoon compote alongside.