Melt butter in a sauce pan.
Place biscuits into blender and pulse until they turn into fine crumbs.
Mix biscuit crumbs with cocoa powder.
Add melted butter and condensed milk into biscuit-cocoa mixture and mix well until you get dough-like consistency. You can add cognac and/or walnuts for added flavor.
Take a little piece of dough at a time and roll it into a ball or oval shaped “potato” to make it look more like a classic version.
Roll each ball in cocoa powder. When all the balls are done, place into refrigerator and let them cool. You can eat kartoshka right away, but it does taste better when you let it cool down.