Popular Eastern European delicious dessert, Tinginys!

Tinginys, often called "lazy cake" or "lazy man's cake," is a beloved Eastern European dessert. 400 grams (about 14 ounces) of plain biscuits (like digestive biscuits or graham crackers or lithuanian gaidelis or polish selgal biscuits) 5 tablespoons of good quality unsweetened cocoa powder 200 grams (about 7 ounces) of butter 1 can of sweetened condensed milk