अथ य इच्छेत् पुत्रो मे कपिलः पिङ्गलो जायेत द्वौ वेदावनुब्रुवीत सर्वमायुरियादिति दध्योदनं पाचयित्वा सर्पिष्मन्तमश्नीयाताम् ।ईश्वरौ जनयितवै ॥६,४.१५ ॥
In plain words
Now he who wishes, "Let a son be born to me who is tawny and brown-eyed, who will recite two Vedas and attain the full term of life" — let the two of them have rice cooked in curd prepared, and eat it with butter. They are then able to beget.
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
दध्योदनं दध्ना चरु पाचयित्वा; द्विवेदं चेदिच्छति पुत्रम्, तदा एवमशननियमः ॥
In plain words
'Curd-rice' is simply the oblation-rice cooked in curds. If it is a two-Veda son a man wants, then this is the diet he and his wife must keep.
Close to the Sanskrit
'Curd-rice' means the oblation-rice cooked with curds; if one desires a son [who will master] two Vedas, then this is the rule of eating [to be observed].