Prepare the dough: Mix flour, cocoa, baking soda and salt in a bowl.
Blend butter with both types of sugar in a mixer bowl with a K-Beater for a uniform mixture. Add the egg and vanilla extract and blend well. Add the flour mixture and blend just until the dough becomes uniform. Keep refrigerated while preparing the filling.
Preparing the filling: Sift powdered sugar into a bowl. Add Biscoff Spread and mix well with a tablespoon or a teaspoon until the mixture resembles a soft, comfortable but not sticky dough. Make 25 little balls from the mixture 3/4 of an inch in diameter.
Assemble the cookies and bake: Form approximately 25 balls from the dough, slightly larger than the filling balls (approximately 1 inch in diameter) and flatten them manually into flat and thin circles. Place a ball of filling at the center of each circle, close off the edges of the dough so that the filling is well wrapped and roll between your hands until a ball is made. Place in the baking pan and flatten to a thickness of approximately ¾ of an inch.
Bake for about 9 minutes, just until cookies lose their shine and their tops stabilize.
Remove and cool. You can store for approximately 3 days covered in a container at room temperature.
For the dough:
1¼ cups of sifted flour
¼ cup of sifted cocoa powder
½ teaspoon of baking soda
A pinch of salt
1 stick (8.5 tablespoons) of soft butter
½ cup of sugar
3.5 oz (½ cup minus 2 tablespoons) of dark brown sugar