1. Lightly grease a 10- or 11-inch fluted tart pan with a removable bottom. Center a rack in the middle of the oven and preheat to 350F.

2. To make the cocoa tart crust: Combine the dry ingredients in the bowl of a food processor fitted with the metal blade. Pulse several times to mix. Add the butter and pulse 12 or 15 times at 1-second intervals until it is finely mixed with the dry ingredients. Add the egg and water, and pulse repeatedly until the dough forms a ball. Invert the dough onto a lightly floured work surface and carefully remove the blade. Form the dough into a rough cylinder. Cut off a third of the dough and set aside.

3. Lightly flour the remaining dough and form it into a disk. Place the disk of dough into the prepared pan and use the floured palm of one hand to press it so that it covers the bottom of the pan. Divide the reserved dough into 3 pieces and roll each into a pencil-thick strand about 10 inches long. Arrange the strands of dough, slightly overlapping at the ends, around the inside of the pan to cover the side. Press the dough against the side of the pan and press it well where it meets the dough on the bottom of the pan. Finally, even off the dough at the top rim of the pan and create a fluted edge, pressing inward firmly with your thumb against the inside of the pan and downward at the top rim with your index finger at the same time.

4. Use a fork to pierce the tart shell all over at 1-inch intervals. Bake for about 10 to 15 minutes or until the shell is dull and dry-looking but still spongy and tender. Transfer the pan to a wire rack to cool.

5. For the ganache filling: Place the chocolate in a medium heatproof bowl set over a saucepan half-filled with just-boiled water. Stir occasionally until the chocolate is melted, about 5 minutes. Remove the bowl from the pan of water and cool the chocolate to room temperature, being careful not to let any water into the chocolate.

6. In a medium saucepan over medium heat, combine the cream and corn syrup. Cook for about 5 minutes, until the mixture is barely bubbling around the edges (about 140F). Pour the cream mixture into a bowl and cool it to 110F. (You will be able to insert a fingertip in the cream and leave it there without a burning sensation.)

7. Pour the cooled cream mixture over the cooled chocolate and use a small whisk to combine until smooth, being careful to not to whisk air into the mixture. Let the ganache stand at room temperature until it is cool to the touch (about 85F). Whisk in the butter a couple of tablespoons at a time. (The butter should be the consistency of mayonnaise, or it will form lumps.) When all the butter has been incorporated, transfer the filling to the prepared tart shell (still in the pan), smoothing the top. Allow 1 hour to set the filling; the tart can be covered with plastic wrap and stored at room temperature for up to 1 day.

8. Just before serving, uncover the tart and scatter the chocolate shavings in the center of the filling. Dust them very lightly with confectioners' sugar. Unmold the tart and slide it onto a platter. Slice the tart with a thin, sharp knife dipped into hot water and wiped after every cut.