Dov Ber the diabetic couldn't control himself when he saw the chocolate danish.
He decided to eat it.
The Rov tells him to make a Brocho Rishono but he shouldn't make a Brocho Achrono.
1) Brocho Achrono needs a shiur .(minimum amount must be eaten)
2) Brocho Rishono has no shiur.(no minimum food need to be eaten)
3) You don't make a Brocho on food that is harmful.
When he bites into the first Mashehu (minimum piece), it is not yet
harmful (too small) , he is mechuyav in a brocho rishono.
After eating a shiur, it is harmful, therefore no Brocho Achrono.
psak from R.Y.S. Elyashiv zt"l