Preheat the oven to 350 F (or 175 C).
Grease a 7 by 10 inch rectangular baking pan, or a 9 inch square pan. In a medium bowl sift together the flour and baking powder, and set aside. In an electric mixer fitted with a paddle attachment, cream together the butter and sugar until very soft and fluffy. Scrape the sides of the bowl with a spatula and continue to mix.
Add the egg yolks, one at a time and mix well before adding the next. Add all the dry ingredients at once, start the mixer at a very low speed for about a minute or two, make sure to not overwork the dough. This is a crucial part of this dough’s quality, it must be crumbs, small ones, almost like sand, the more you’ll work at it the more it will get “wet” and loose the sand effect. (if you have a few pieces of butter that didn’t mix well or a few big crumbs don’t fret, it will all work out).
Pour a little more then half the dough onto the baking pan, make sure it’s an even layer, don't press it, let it stay loose.
Pour over it the jam, you can do it with a big spoon and then try to spread it a little to all corners with the back of the spoon. Pour the rest of the dough on the jam, spread to even the layer and bake until the crumbs on top are golden brown about 35 minutes.
Take the pan out of the oven, put it on a cooling rack, and while the cake is still hot cut it with a sharp knife into squares to make bars, any size you like. I like to cut them into small cookies. Let it cool completely before storing it in an airtight container.