This recipe is based on my recollection of a Gourmet Magazine recipe for stuffed French Toast. I haven't found the original on the website, but this is how I usually make it. I added light cream because it was in the fridge and it seemed like a good idea. The sauce is based on a recipe for lemon (cake) filling in Joy of Cooking, 25th printing, 1981, with some Meyer lemon juice and zest added, and the sugar amount reduced. —creamtea
day-old loaf braided challah (about a 20 oz. loaf)
milk (I used 1% but whole milk would work as well)
Slice challah on the diagonal into thick (1 to 1 1/2 inch) slices and cut the larger-width slices in half vertically (i.e., from point A at the the braided edge to point B at the flat edge)
Break eggs into a medium bowl and whisk until foamy. Add next 6 ingredients and whisk briskly to combine. Set aside.
Insert the blade of a thin sharp knife into the center of the narrow cut edge of each challah slice to pierce a slit suitable for insert chocolate (that is, the edge that was made when you cut each slice in half). Keeping the slit small, jiggle the pointed end of the knife somewhat to enlarge the further end of the opening into a pocket. Try to avoid breaking through the bread so that the chocolate won't leak during cooking.
Break or cut smallish pieces of chocolate. Each piece should be less than an inch in width. Insert a piece of chocolate into each slit. Scharffen-Berger breaks into triangular shards, which is helpful in working the bits of chocolate in. Try not to overstuff the pocket or you will end up with a chocolate mess in your pan. You can pinch the slit a bit to secure the chocolate in its pocket.
Whisk the egg mixture to re-combine, and pour a little less than half into the bottom of an 11 x 15" Pyrex baking dish. Arrange the challah slices on top of the egg mixture and pour remaining egg mixture over top. Flip each slice to the other side to more fully absorb the egg mixture. Allow to sit for at least 10 minutes.
In a large saute pan, over medium heat, melt about one tablespoon or more--your choice--of butter, swirling until pan is coated. When butter is frothy and completely melted, carefully add some of the challah slices. Do not crowd pan. Cook until nicely browned, and flip to brown second side--probably about 5- 8 minutes per side. For subsequent batches, add more butter, because more is always better. If making ahead, transfer slices as cooked to a large rimmed baking sheet and keep warm in a slow (150º or 200º F.) oven.
For the syrup, combine well the cornstarch, sugar and salt in the top of a double boiler. Stir well to combine and to break up the cornstarch lumps.
Slowly add the water, lemon juice and rind.
Slowly add water, lemon zest and juice, and butter and place over the bottom of the double boiler, over but not touching the simmering water.
Stir constantly for 5 minutes, then cover and cook over low heat for 10 minutes.
Remove from heat, carefully separate top and base of double boiler. Remove cover, stir to combine, scraping the bottom (some of the sauce mayl have thickened at the bottom of the bowl or pot), and cool slightly for about 1/2 minute. Slowly stir in egg yolks, whisking. Place back on base of double boiler, return to low flame. Cook, stirring, two minutes longer. Remove from heat, stirring occasionally and allow to cool.