Line a full-size sheet pan with a nonstick rubber mat and fit the inside of the pan with a frame .75 cm / .25 in by 53cm / 21in by 33cm / 13 in.
Cream the butter, sugar, vanilla beans, and orange zest on medium speed using a paddle attachment for 6 minutes, or until soft and airy. Gradually add the eggs in 4 additions. Scrape the sides of the bowl after each addition of eggs.
Sift the dry ingredients together and add to the bowl, mixing on low speed until just incorporated.
Fold in the crème fraîche. Spread onto the prepared sheet pan, evening out the batter with the top of the frame. Remove the frame and bake until the cake springs back when touched and has a golden brown color, 10 to 12 minutes.
Cool to room temperature. Freeze for 2 hours before cutting, in order get a clean cut.
Place the frame back over the cake and even out the top by cutting it flush with the frame with a serrated knife.
Flip the cake over and spread the melted chocolate on it, thinly and evenly. Flip the cake back over and cut into rectangles 2.5cm / 1in by 16.25cm / 6 in.
Reserve frozen in an airtight container under refrigeration. This can also be held frozen for about 2 months.