Set your oven to 392°F / 200°C so it’s ready when the cookies are assembled. In a bowl, combine the flour, baking powder, and salt. Add the lard and sour cream, then mix everything together until the mixture comes together. At first, it will look crumbly and a bit rough, which is exactly what you want.
Turn the dough out onto a lightly floured surface and knead it for a few minutes, just until it forms a smooth, compact dough. If you used a mixer with a kneading attachment, you can skip this part.
Wrap the dough and let it rest in the fridge for about 20 minutes so it relaxes and becomes easier to work with.
While the dough rests, prepare the filling. Blend the cracklings until you get a spreadable mixture. If needed, add a spoonful of lard to loosen it. Taste and adjust with a little salt if necessary.
Take the dough out and divide it into two portions. Roll out one half on a lightly floured surface to about 0.16 inch (0.4 cm) thickness.
Cut the dough into evenly sized squares using a knife, pastry cutter, or pizza wheel. They don’t have to be perfect, just similar in size.
Trim any rough edges and keep those scraps for later. Add a small spoonful of filling to each square, keeping it centered.
Fold each square diagonally into a triangle. Press the edges firmly with a fork to seal them well and keep everything inside.
Arrange the cookies on a baking tray lined with parchment paper. Repeat the same process with the remaining dough and filling.
Beat the egg until smooth, then brush each cookie. This helps them hold their shape and gives them a nice golden finish.
Scatter sesame seeds over the top.
Bake for about 13 minutes, until the cookies turn golden and crisp.
Let them cool slightly before moving them. They’re soft and fragile right out of the oven but firm up as they sit. Transfer to a rack and leave until fully cooled.
Serve them up with a cup of tea or coffee and enjoy!