TL: Marisa: So good! This shit's good! tamanne! (not sure what this means) Reimu: Reimu will eat Mister sweet-sweets! ko Reimu: Mister sweet-sweets can take it easy! baby Marisa: H-happinesh!
In this case I'd translate "tamanne" to "(this shit is) irresistible"
more generally, it's a bastardized "tamaranai" which means "I can't stand it" whose meaning would vary depending on context. In this case "it's so delicious that I can't stand it"