Using Element API to output Super Table fields
Here is a solution I found. Define an empty array, then loop over it, providing whatever keys you want to return from the Super Table fields. Simply by looping over $block-&gt;columnTextBlocks (the handle of my Super Table field), Element API returns the information for the fields within the Super Table. 

 case 'twoColumnText':

    $SuperTableRows = [];

    foreach ($block-&gt;columnTextBlocks as $row){
        $SuperTableRows[] = [
            'fieldHandle1' =&gt; $row-&gt;fieldHandle1,
            'fieldHandle2' =&gt; $row-&gt;fieldHandle2,
            'fieldHandle3' =&gt; $row-&gt;fieldHandle3
        ];
    }

    $contentBlocksArray[] = [
        'twoColumnText' =&gt; [
            'columnTextBlocks' =&gt; $SuperTableRows,
        ]
    ];

break;


If you need additional information about the Super Table fields, such as Placeholder text, field id's, etc. you can add a generic call to the Super Table field like so (using the same names as my previous code example):

$contentBlocksArray[] = [
    'twoColumnText' =&gt; [
        'generic' =&gt; $block-&gt;columnTextBlocks,
        'columnTextBlocks' =&gt; $SuperTableRows,
    ]
];


Now in Element API, the "generic" object includes a customFields array for each field within the Super Table. This only becomes available when you loop over the Super Table field, otherwise it would be null.

Here are some pictures of what the data from Element API looks like.





Here is a simplified version:

'transformer' =&gt; function(Entry $entry) {
   $SuperTableRows = [];

   foreach ($entry-&gt;superTableFieldHandle as $row){
     $SuperTableRows[] = [
       'fieldHandle1' =&gt; $row-&gt;fieldHandle1,
       'fieldHandle2' =&gt; $row-&gt;fieldHandle2
     ];
   }


   return [
     'SuperTableRows' =&gt; $SuperTableRows
   ];
 },