Grind the biscuits in any way that works for you. You can use a large food processor if you have one, a meat grinder like in the old days, or simply a rolling pin. Place the biscuits in a sturdy bag and crush them well; the crumbs can be slightly uneven and a bit chunky – this will not hurt the taste at all.
Add the softened butter and condensed milk to a bowl. Mix them together with a fork until you get a smooth cream. You can use a mixer if you prefer, but it is not necessary for this recipe, because the cream does not need to be perfectly whipped. Using only a fork or whisk also means you have fewer dishes to wash.
Tip the biscuit crumbs into the butter and condensed milk mixture. Stir everything together with the same fork until the crumbs are fully coated and you get a dense, moldable dough. The mixture should hold its shape when pressed between your fingers.
Sprinkle cocoa powder onto a flat plate. Take portions of the dough and shape them into small, elongated cakes that look like mini potatoes. Roll each piece in cocoa powder on all sides, then place the formed “potatoes” onto a sheet of baking paper or a tray. You can make them more oval or round, depending on how you like them to look.
Transfer the tray with the chocolate potatoes to the refrigerator for at least 1 hour. During this time the butter will chill and firm up, so the candies become more compact and easier to handle. Serve the kartoshka cold, straight from the fridge, for the best texture and flavor.