엣 is an old-fashioned squeeze of 에 있는 'the one that is at', and here it lives in two nouns: 위엣것 'the one on top' and 밑엣것 'the one underneath' — 위엣것 다 먹었으면 이제... 'once you've finished the top layer...' It sounds folksy and is mostly used for stacked food or piled objects. Several lines here (하이엣 for hi-hat, 에엣 for the count 'eight', 시장엣서) merely contain the syllable.