Preheat your oven to 175˚C or 350˚F. Line your baking sheet with baking paper.
In a medium sized bowl, sift together flour, baking powder, salt and cocoa powder.
In another bowl, using a hand or a stand mixer, mix together butter and sugar until pale and fluffy (approx. 2-3 min)
Add an egg and vanilla extract and mix until everything is combined.
Add your dry ingredients to butter, sugar and egg mixture in TWO batches, mixing just until everything is incorporated and there are no lumps of flour. You don't want to overmix it, otherwise your cookies will get tough.
Divide it in two, wrap up in cling film forming a disc and put it in fridge for an hour or up to a week.
Once you're ready to bake the cookies, leave the dough at room temperature for 15 to 20 minutes, until it's ready to roll.
On a floured surface roll out the dough to a ¼ inch thick circle and cut out 2 inch wide circles. There should be around 20-25 cookies. Using a smaller cookie cutter cut out the middle in half of your cookie circles. I used a small heart cookie cutter but you can use whatever you want, even a round nozzle! :)
Bake the cookies for about 10 minutes. They should be lightly soft in the middle and crisp on the outside. Transfer them to a cooling rack to cool completely.
Dust half of your cookies (the ones without the middle) with powdered sugar. Put a teaspoon of jam in the middle of each cookie but be careful not to spread it all the way to the end, so it doesn't ooze out when you sandwich the cookies. Place the other cookie on top and enjoy!
Not all ovens are the same. I suggest baking a couple of "test cookies" to find out the best baking time or temperature. Get to know your oven!
If you still don't own one, I advise you to buy a kitchen scale - they're cheap and easy to use, and always guarantee the same results in baking!