For the alfajor dough:
Put the butter and sweetener in a bowl and mix both ingredients with a fork until the butter becomes creamy.
Then add the egg and lemon zest to the bowl. Stir until well mixed and then add the flour, cornstarch, and baking soda, previously sieved. Mix well with a spatula to form the dough.
Once ready, knead the dough with your hands until obtaining an easy-to-handle dough that does not stick to the bowl.
Make a ball and set aside for 30 minutes. Preheat the oven to 400°F.
After 30 minutes, flour a baking dish, take one third of the dough, make a ball, and smash it. Using a rolling pin, make it 0.2-inch thick and cut the dough with a round mold.
Put the circles in the baking tray with flour and repeat with all the dough. Put the tray in the oven and bake for 15 minutes or until the edges of the alfajores starts to brown. Remove from oven and allow the biscuits to cool.
For the assembly: take one biscuit, put some dulce de leche with a spoon, 3 raspberries, and another biscuit on top to cover and form the alfajor. Repeat until making all the alfajores.