The month of May is often associated with the names of egg and dairy dishes. With cattle back on pasture, chickens laying, and the fast days of Lent over, these were the seasonal delicacies. May cake (Meyscher Kuchen) is a frequently mentioned preparation, basically just egg cooked solid in a container. This recipe has an almost verbatim parallel in Cod Pal Germ 551 as well as in Meister Eberhard, as we will see when I get to that source.
[] Wilthu machenn ein guettenn weyschen Kuchen:
If you want to make a good May (read meyschen for weyschen) cake
Take up to ten eggs, break them well, add parsley and stir it all together. Take a mortar, place it on the embers, and put a spoonful of lard in it to heat. Pour the eggs into it and bake it at a low temperature and serve it, not oversalted.
Another instance of using the mortar for cooking with a slow and steady heat, but otherwise an unremarkable recipe.
The Königsberg MS was preserved in the archive of the Teutonic Order in Königsberg (today Kaliningrad) in Baltic East Prussia, though its language suggests that it belongs to a Central or South German background. It is not associated with any name that I am aware of and is dated to the late 15th century purely on the scribal hand. The recipe types match South German sources of that time. It was published in Gollub (Hg.): Aus der Küche der deutschen Ordensritter. in: Prussia 31 (1935) pp. 118-124.