My pantry currently contains a Lindt bar that says Edelbitter Chocolade on the wrapping. I wonder if this is spelling of Schokolade is just a fancy of that particular manufacturer, or a quite usual spelling, or somehow significant.
Here is a ngram for Schokolade/Chocolade and Chocolat:
You can see Chocolade (and Chokolade) is an old writing, it was the common writing before 1900. As a trade name you can give the impression of a traditional piece of chocolate.
To answer your question:
I wonder if this is spelling of Schokolade is just a fancy of that particular manufacturer, or a quite usual spelling, or somehow significant.
It is no usual spelling. Only Schokolade is correct. But it is a common writing of chocolate producer to give there article a special image.
The spelling "Chocolat" is the spelling used in French language so the spelling "Chocolade" is similar to the French one.
In Germany many companies who try to give their products (mainly food and cosmetics) the reputation of having a higher quality use French words instead of German words or they use a spelling which is similar to French.
Another very common example for using French words would be pastry cooks who whose the word "Confiserie" instead of the German word "Konditorei" for their pastry shop.
Lindt is one of the companies often advertizing their products as "higher-quality" products. (I remember an advertising spot in TV where they claimed that they need more time for the production than the competitors because this is needed for a special taste of the chocolate.)
"Chocolade" is a French word with a German ending (ade). The original French is "Chocolat."
A purely German spelling (preserving the sound), is "Schokolade." That is, the "ch" would be accompanied by an s, and the second c would be replaced by a k.
But the user of the term "Chocolade" wanted to combine both the French and German elements, preserving the original French "c"'s, while using the -ade ending to signify that the word had been Germanized to some degree. This was probably done for "chicness." (Note the French "ch" and "c" in the last word.)