Add unsalted butter, sugar, and salt in a big bowl. With the hand blender, beat them on medium speed for 2 - 3 minutes. Mix them properly till well combined and fluffy.

Now put the dough on a plastic wrap. Mix it lightly with your hands to a cohesive dough. Get it rolled into a log and wrap it completely. Store the wrapped dough in the fridge for 30 - 40 minutes.

Preheat the oven to 350 C or 177 F. In the meantime get the wrapped dough out of the fridge. Cut it into 24 pieces. Take each piece and place it at the bottom of each tart shells in the pan. Press each dough gently and evenly against every side and bottom. With a fork pierce at every bottom of the tart shells.

Bake for 15 - 20 minutes or till it gets lightly golden brown. Cool these tartlet shells in the pan for 15 minutes. Let the tart shells cool completely on a wire rack.

First, for garnishing keep 12 raspberries aside. Then add the remaining raspberries and sugar to a small pan. Take the potato masher and mash the strawberries to a pulp. It will release their juices.

Keep the saucepan on medium-low heat. Cook for nearly 10 minutes stirring occasionally. Once cooked, take the mesh sieve or strainer and strain the raspberry sauce in a bowl. Let it cool down.