chocolate bar; bar of chocolate
Refers to a solid, flat bar of chocolate, typically the kind sold in blocks for baking or snacking.
板チョコを割ってクッキーに入れた。
I broke up a chocolate bar and put it into the cookies.
Compound of 板 (ita, 'board, slab') and チョコ (choko, abbreviation of チョコレート 'chocolate').