How to Make It
In a food processor, pulse the flour with 2 tablespoons of the sugar, the cocoa and the salt. Add the butter and pulse until the crumbs are the size of small peas. In a small cup, stir the vanilla into 3 tablespoons of ice water, then sprinkle over the mixture and pulse just until the crumbs are moistened. If the crumbs seem very dry, pulse in the remaining 1 tablespoon of ice water. Turn the crumbs out onto a work surface and gather them together; pat the pastry into a disk. Wrap the pastry in plastic and refrigerate until chilled, about 30 minutes.
Preheat the oven to 400°. On a lightly floured surface, roll out the pastry to a 14-inch round (see Note). Transfer the round to a baking sheet lined with parchment paper. Fold 1 inch of the pastry over onto itself to form a pleated rim and prick the rest of the pastry with a fork. Brush the rim with egg white and sprinkle with the remaining 1 teaspoon of sugar. Freeze until firm.
Bake the pastry for 20 minutes, or until it's nearly cooked through and dry to the touch.
Using a vegetable peeler, remove two 3-by-1-inch strips of zest from the orange. Cut the strips into fine julienne with a sharp knife.
In a large nonstick skillet, melt the butter over high heat. Stir in the sugar, orange zest, rosemary and vanilla. Add the plums and water and cook, stirring occasionally, until the sugar dissolves and the plums are barely softened, about 5 minutes.
Spread the cooked plums and their juices over the pastry and bake for 35 minutes, or until the fruit is tender and the crust is cooked through. Cut the galette into wedges and serve warm or at room temperature, with whipped cream.
For a different look, roll out the pastry into a square or rectangle.