There are many and varied recipes for Tarta de Santiago available on the internet. I have also seen recipes in many cookbooks while in Spain, albeit in Spanish...doh! Of course!
The tricky bit is making the shape of the Cruz de Santiago on top, before you dust with confectioner's sugar, or some other suitable topping.
When I first persuaded my mother (a world class baker) to try a tarta, I provided a really good recipe from the internet. I also printed a Cruz de Santiago on heavy paper and cut it out. It worked great, except the cross was a lot of delicate scissor work for a single use.
That was in 2015. I have since found really good inox (stainless steel) Cruz de Santiago templates. They are sold in ferreterias and cooking supply shops all across Santiago.
Typically, they come 2 to the set. One is perhaps 4 inches tall. The other is about 8 inches tall.
I have bought them for everyone I knew who bakes. But, I am always looking for more.
They make outstanding gifts for anyone with a Camino fixation. They are also good to buy for a friend, spouse, family member or S/O who does your baking.
I gave away the set I had at home in Florida to a Camino colleague who does more baking than we do. My mom hangs her set as wall and window ornaments when not being used. They cost about €8 for the set.
Also, this year I found smaller red, plastic Cruz de Santiago molds for baking. They were in a souvenir shop opposite the Pilgrim Office. They cost about €3 each.
If anyone had found the stainless (Inox) version online, can you share the link? Thanks.
Hope this helps.