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Donald Trump Jr. crossed the line of decency by posing with a giant cookie frosted with a Barack Obama caricature ... this according to a pissed off Dallas restaurant owner.

Stephan Courseau owns Le Bilboquet, the French eatery where the photo op went down, and he tells TMZ ... he never would have allowed the cookie to see the light of day. He says the frosted image -- attempting to recreate Obama's "Hope" posters -- is laced with racist overtones.

Some critics have said it looks like a "racist minstrel show on a cake of Obama."

Jr. held up the cookie while posing with Ted Cruz, and it drew immediate backlash. Courseau says his regular customers flooded him with calls, asking how he allowed this to happen.