こう見えて、子供の頃はケーキを二ホール平らげた伝説を持っている。

、。

Believe it or not, when I was a kid I have the legendary feat of polishing off two whole cakes.

Literal

Like-this appearing, child [possessive-の] time [topic-は] cake [object-を] two-whole finished-off legend [object-を] possessing.

こう見えて 'despite looking like this, believe it or not' — a set phrase for surprising confessions. ホール is a loanword for 'whole (cake)' — a counter-like usage. 平らげる 'to polish off, to eat everything' is vivid. 伝説を持っている 'possesses the legend' humorously treats a childhood eating feat as mythology.