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1
Blend flour, powdered sugar and salt in processor 5 seconds.
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2
Add butter and cut in using on/off turns until mixture resembles coarse meal.
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3
Mix in yolk and enough water to form moist clumps.
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4
Knead dough briefly on work surface to combine; flatten into disk.
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5
Wrap in plastic and refrigerate until firm, at least 2 hours and up to 1 day.
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6
Preheat oven to 375F.
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7
Roll out dough on lightly floured surface to 16-inch round (scant 1/8 inch thick).
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8
Transfer dough to 11-inch-diameter tart pan with removable bottom.
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9
Press dough into pan; trim excess.
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10
Freeze crust 15 minutes.
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11
Place tart pan on baking sheet.
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12
Bake crust 10 minutes; pierce with toothpick if crust bubbles.
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13
Continue to bake until crust is pale golden, about 12 minutes.
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14
Transfer to rack; cool crust completely.
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15
Stir chocolate, cream and espresso powder in heavy medium saucepan over low heat just until chocolate melts and mixture is smooth (mixture will be warm, not hot).
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16
Pour mocha ganache into prepared crust.
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17
Stir white chocolate and cream in heavy small saucepan over very low heat just until chocolate melts and mixture is smooth.
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18
Drizzle over mocha ganache.
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19
Using tip of small knife or toothpick, swirl fillings together to marbleize.
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20
Refrigerate tart until filling is cold and firm, at least 3 hours.
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21
(Can be prepared 2 days ahead.
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22
Cover and keep refrigerated.)
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23
Push pan bottom up to free tart from pan.
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24
Cut tart into wedges and serve.
