Preheat your oven to 175˚C (350˚F). Grease and line with baking paper 10 inch (25 cm) square tin or 9x13 inch (22x33 cm) tin. Either will work just fine.
In a medium sized bowl, beat together butter and sugar using a hand mixer, until lighter in colour and combined. Add egg yolks and beat until incorporated.
Next, sift together flour and baking powder, then add it to the butter mixture. Mix on low with hand mixer until it becomes crumbly, then knead it a little bit with your hands until all comes together.
The dough will be very soft. Divide it in two, with one part being just a little bit bigger than the other. Wrap the bigger part in cling film forming a disk of dough and place it in the fridge for an hour. That's the base of your pie bars. The smaller part just wrap in cling film, don't flatten it or form a disk, and put it in the freezer. You'll use that to grate it on top of the filling.
for the yogurt lemon filling
Clean the bowl of your stand mixer (or any stainless steel bowl if you're using a hand mixer) using some vinegar/lemon juice and a paper towel. Do the same for the whisk attachment.
Place the egg whites in the bowl and mix on medium speed until it becomes frothy, then start to add spoonful by spoonful of caster sugar. When you're done adding sugar, whack the speed on medium high and mix it until you get a stiff peak meringue, just a couple of minutes. You don't want to overmix the eggwhites, so they end up split.
In a medium sized bowl, combine yogurt, sour cream, all purpose flour, lemon zest and vanilla extract using a whisk. Then pour that mixture over the meringue and fold everything carefully (you don't want to lose the air beaten into meringue) until it incorporates into smooth filling.
Roll the dough from the fridge between two sheets of baking paper (or on a lightly floured surface if you prefer it that way) to fit your pan, then transfer it to the pan. There's a big possibility that the dough will crack and tear when you try to transfer it to your pan. That's okay. Just patch it with your fingers - I do it all the time!
Pour the prepared filling evenly over the dough. Grab the leftover dough from the freezer and grate it all over the filling.
Bake in a preheated oven for 35-40 minutes, until the top is golden and a bit puffed up. When it's done, it will be slightly wobbly, just a bit, that's fine.
Leave it to cool in the pan to room temperature, then transfer it to the fridge to cool completely before serving.
Keep these pie bars in the fridge in an air tight container or in a pan you baked it in, but wrapped in cling film. They're the best straight from the fridge - refreshing!
If you don't have a stand mixer, you can use hand mixer.
If you don't have thick Mediterranean style yogurt, you can use greek yogurt instead. Just don't use liquid, drinking type of yogurt.
If you still don’t own one, I advise you to buy a kitchen scale – they’re cheap, easy to use and always guarantee the same results in baking!