As delicious as this tart is with blood oranges, regular navel oranges are a tasty substitution.
To prepare oranges, slice thinly on a mandolin (or with a serrated knife), and blanch for 1 minute in boiling water. Strain and cook in sugar with 1 cup fresh water for about 15 minutes, until pith takes on a slightly clear appearance. Drain and cover (to prevent drying out), reserving 2 Tbsp of the liquid for mixing into tart filling.
For pastry, cream butter and sugar together until light and fluffy. Beat in melted chocolate, egg yolks, vanilla and orange zest. In a separate bowl, sift together flour, cocoa powder and salt. Add to butter mixture and mix just until dough comes together. Wrap and chill for one hour.
Preheat oven to 350° F. On a lightly floured surface, roll out pastry to just less than 1/4 -inch thick. Line pastry into a 9-inch removable-bottom tart shell and trim edges. Chill for 15 minutes. Dock bottom of shell with a fork and bake for 15-18 minutes, until pastry takes on a dull, dry appearance. Allow to cool.
Reduce oven temperature to 325° F. Place chopped chocolate in a large bowl. Heat cream with orange zest to just below a simmer and pour over chocolate. Let it sit for a minute then stir slowly. Stir in reserved orange syrup, vanilla and orange liqueur. Whisk egg in a small cup then stir into chocolate. Pour into chocolate shell. Arrange candied orange slices over chocolate filling and bake for 12 minutes. Let cool for 15 minutes, then chill for 2 hours, before slicing.
To serve, slice with a hot, dry knife.
Yield: 1 9-inch tart