Betty Botter bought some butter;
"But," said she, "this butter's bitter!
If I put it in my batter
It will make my batter bitter.
But a bit of better butter
Will but make my batter better."
Then she bought a bit of butter
Better than the bitter butter,
Made her bitter batter better.
So ´twas better Betty Botter
bought a bit of better butter.

Betty Botter bought the bitter butter, but she realized it was bitter before she put it in her batter ("IF I put it in my batter..."). Why then did she have a bitter batter after she bought the better butter? Her batter should not have been bitter because she did not put the bitter butter in. OR DID SHE?
What if she was just using the butter as an excuse for her bitter batter? In fact, she may have added poison to her batter so it became bitter. However, she didn't want people to be suspicious so she had to buy some more butter to cover up the taste.
Notice we are told that she "bought a bit of better butter", but "better" in what sense? Maybe it was just "better" at covering up the bad taste of the poison. The "better batter" was just a poison-laced batter that was better concealed.

Who was Betty Botter trying to poison with her batter?