Beat butter and sugar until fluffy. Add egg and salt and beat until creamy.
Mix flour and baking powder and add little by little.
Divide dough into two equal halves.
Knead one half with cocoa, the other with tahini.
For the marble look, divide the doughs into small pieces and roll them back into one large one until the light and dark doughs have twisted into a pattern.
Shape the dough into a roll about 30 cm long. If the dough is too soft, you can wrap it in cling film and put it in the freezer for 10 minutes. Then it can be worked better again.
Cut the dough roll into slices about 2 cm thick and place them on a baking tray lined with baking paper. Be sure to leave enough space between the slices, because they will still separate during baking.
Bake marble cookies for about 10-15 minutes and then let cool.
For the glaze
Melt white chocolate over a water bath and mix with tahini.
Using a spoon, dip into the glaze and pipe thin strands over the cooled cookies.