The THC and fat from the chocolate are both metabolized, so there's no reason why smoking weed and eating fatty foods would affect each other in any way.

Now, eating a bunch of fatty foods and chasing it with pot brownies might prolong the onset of the drug, but wouldn't reduce the strength.

Oh, and chocolate isn't made from milk, it's made from cocoa, butter, and sugar. They just remove the cocoa liqueur and leave the cocoa butter when it's so-called "milk chocolate".