Instructions: Wrap underside of a 7 inch spring form pan tightly with heavy-duty aluminum foil. Butter inside of pan with melted butter. Sprinkle the unwrapped bottom of pan with cookie crumbs.

In top of a double boiler over hot (not simmering) water, melt chocolate, stirring occasionally, until smooth. Remove top of double boiler from heat and let the chocolate cool until tepid.

In a medium bowl, using a hand-held electric mixer at medium speed, beat cream cheese and sugar until smooth. Beat in sour cream, flour, vanilla and orange zest. Beat in eggs and egg yolk.

Pour about 1/4 of cheese mixture into prepared spring form pan. Pour another 1/4 into a small bowl and set aside. For second layer, stir cooled chocolate into remaining vanilla mixture until well-blended and pour into pan. For third layer, pour remaining vanilla mixture over chocolate in a swirl pattern. Run a knife through to make a marble pattern.

Butter a 15 inch-long piece of aluminum foil. Tightly cover entire pan with the foil, placing buttered side down, allowing some room at the top for the cheesecake to rise. (The foil should completely cover the pan.)

Place your pressure cooker trivet in bottom of a 5- to 7- quart pressure cooker. Pour 2 cups of water into cooker. Center covered dish in an aluminum foil `cradle. To make cradle, tear off a 24 inch length of aluminum foil. Fold strip lengthwise in half, then in half again, to make a sturdy strip of foil about 3 inches wide. Place dish on top of strip and use ends of the strip as a handle to lift pan in and out of pressure cooker.

Lock cooker lid in place. Bring to high pressure over high heat. Adjust heat to maintain the pressure. Cook 25 minutes. Remove from heat. Allow pressure to drop naturally, about 20 minutes. Open lid, tilting it away from you to block any escaping steam.

Using cradle, lift dish out of pot. Let stand 2-3 minutes. Remove foil. If necessary, use paper towels to blot any water on surface of cheesecake. Run a sharp knife around inside of pan to release cheesecake from sides. Place on a wire cake rack and cool completely.

Remove sides of pan. Cover cheesecake with plastic wrap and refrigerate until well chilled, at least 4 hours. Serve chilled, using a sharp knife dipped in hot water to cut each slice.

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