Preheat oven to 180ºC.
Cream butter and sugar very well.
Add vanilla and beat in the condensed milk.
Add baking powder and sifted flour gradually.
If the dough is very stiff, do not add all the flour.
Divide mixture in half. Add the cocoa to one half.
Roll out each half on a sheet of tin foil until about 6 mm thick and 40 x 24 cm big.
Place the white layer on top of the brown layer.
Roll up like a swissroll.
Allow to chill for 30 minutes.
Cut slices 6 mm thick, place on a baking tray and bake for 10 to 12 minutes until light golden.
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