Chink in one’s armour = A minor but very detrimental flaw or weakness.

• Yeah, he’s brilliant, but his violent temper has destroyed many business relationships.  It’s really the chink in his armour.
• Putting things off to the last minute is the chink in Mary’s armour and is bound to get her in trouble one day.

Whet one’s appetite =  to awaken a desire/ increase the feeling/stimulate appetite or desire; to sharpen the blade of( a tool or weapon).

• Cinemas use trailers to whet viewers’ appetite and make them want to see the whole film.
• She took out her dagger and began to whet its blade in even rhythmic strokes.
• I have been listening to V.R.Dilipkumar’s classical concerts for many years and  it whetted my appetite for Carnatic raagas.

The icing on the cake = an additional benefit or positive aspect to something that is already considered positive.

• Having all of you here for my birthday has really been wonderful!  This gift is an icing on the cake.
• The athlete received a gold medal and the icing on the cake was that he also received a sponsorship deal.