Mix together the sifted flour, salt, baking powder. Separately, beat the soft butter with both types of sugar until you get a fluffy cream. Add the eggs one at a time, mixing well after each. Add the rum essence and cocoa. After the mixture becomes homogeneous, add the floury mixture.
Leave thus prepared dough in the refrigerator for 1 hour, with the bowl covered with foil. Divide the dough into 2 parts and while working with one, keep the other in the refrigerator, because you have to work with a cold dough, so it doesn't stick. Roll out a layer (about 1/4″ (1 cm)) on a lightly floured surface.
Cut out biscuits of the desired shape. You don't have to use forms, you can simply cut them with a knife. Arrange the brownie biscuits in a tray with baking paper some distance apart from each other. Put them to bake for about 10 minutes in a preheated 356°F (180 °C) oven.
After the 8th minute, be careful not to burn them. They are ready if they bounce slightly after being poked in the middle. If they are overbaked, they will become dry and crumbly.
Let the finished brownie biscuits cool and pour on melted white chocolate for decoration.