For the dough: Place flour in a mixing bowl and sift the cocoa powder over it. Stir in the sugar, salt and baking powder. Rub in the butter finely, leaving the mixture cool and powdery. Beat the egg and stir it into the dough. Press the dough together and wrap and chill it.
Preheat oven to 350 degrees and set rack in the lower third of oven. On a floured surface, roll the dough and line a buttered 10-inch tart pan. Set aside while preparing filling.
In a saucepan, over medium heat, bring sugar and water to a boil. Add butter and continue heating to melt butter. Off heat whisk in finely cut chocolate. Whisk eggs with spices, then whisk in chocolate mixture. Pour into tart shell.
Bake for about 30 minutes, until well risen and firm. Cool on a rack. Unmold tart and serve with sweetened whipped cream.