Grate the zest from the orange then cut and half and juice it - you should get around ¼ cup (60ml) or slightly more. Melt the butter and set aside.
Prepare a 7 x 9in baking sheet/tray that's about 1in deep (19 x 23cm, 2.5cm deep) by lightly greasing with butter and lining with parchment paper.
Place the egg and sugar in a medium bowl and whisk them together (you can also use a stand mixer). Mix for a few minutes until the mixture becomes pale and glossy with small bubbles in it.
Add the orange zest, juice, vanilla paste and melted butter and gently stir to mix together.
Add the flour, yeast and salt and mix so everything is well combined. Add the water - you may not need quite all if you had more than ¼ cup (60ml) orange juice - and mix in. The dough should be relatively smooth and soft but not runny.
Transfer the dough to the prepared baking sheet/tray and spread the dough evenly to the edges. Smooth the top, using your fingers as needed. Cover with cling wrap/film and set aside in a place at warm room temperature for around 2 hours. It won't rise that much but will become a little more puffy looking.
Preheat the oven to 375F/190C. Remove the cling wrap/film and bake for around 25 minutes until gently brown on top and dry to touch. You don't want it to get too brown or it will dry too much around the edges.
Let the cake cool around 5 - 10 minutes then dust with sifted powdered sugar (icing sugar) all over the top. Leave to cool almost completely.
As it's cooling, make a fleur-de-lys stencil from parchment. Once the cake is relatively cool, lay it over the cake and dust cocoa powder into the stencil shape. Carefully lift up the stencil to leave the shape in cocoa powder.