Most cookies in Kingdom very clearly state there flavor in there name like Mint Choco and Raspberry cookie. Some cookie's have a title instead of a flavor- and I think I figured all of them out.
Vampire cookie: Grape Juice- his profile says so. The same goes for his Sister, Alchemist who is cannotically grapes.
Sparkling cookie: This man is carbonation. I've seen a lot of people say he's champagne which may not be cannon due to universal censorship laws preventing alcoholic cookies and an Kumiho depiction in the games. Sparkling's cannon flavor is probably sparkling lemonade
Wizard cookie: Looking at his hat and hair he's probably a vanilla ice cream cone
Cream Unicorn cookie: since his OB pet is called meurange {< I can't spell that to save my life} horn we can assume he may be some kind of creamy sugary meurange.
Cyborg cookie: Probably tastes like a bunch of sugary computer parts and I don't think Cyborg cookie is technically a cookie- he was a robot built by Aloe cookie sooooooo
Carol cookie: If anyone watches Food Theory on youtube you may be familiar with a video about using a christmas tree to make cookies {as depicted in the thumbnail} yeah Carol cookie tastes like a christmas tree
Kumiho cookie: Mashmellows- her fox form is described as being a mashmallow fox
Millenial tree cookie: LEAVES! TWIGS! GRASS! Man probably tastes like an entire forest
Fire Spirit cookie: He tastes like Ghost peppers- that's where the SPIRIT comes from in his name
Angel cookie: There flavor is likely similar to Angel's Food Cake so if you were to eat this cookie you'd taste egg whites, vanilla, and a lot of SUGAR
Devil cookie: While I personally think this cookie tasting like Devil's food cake would be perfect- the amount of carbonation they mention seems to point towards them tasting like soda
Pastry cookie: This was the hardest to track down. Since she wears a lot of purple I looked into purple pastries and while I didn't find a proper answer I found a recipe for Purple Velvet Cupcakes which are topped with white frosting. Just imagaine the irony of Pastry cookie's nemesis, Red Velvet cookie finding out she's a Purple Velvet flavor.
{honestly though Pastry cookie likely a kind of french bread like brioche. French bread because France is very religious and you can pretty much find pretty churches all over the place. Pastry cookie seems to have been inspired by Catholics, which is a major religion in France}