1. Log in to your dashboard--> Design- -> Edit HTML

2. Click on "Expand Widget Templates"

3. Search "Ctrl+F" and find the following code:

<div class='art-postcontent'>

replace it with:

<div class='art-postcontent' expr:id='&quot;post-&quot; + data:post.id'>
<b:if cond='data:blog.pageType == &quot;item&quot;'>

4. Find:

<data:post.body/>

and right after it paste the next code:

<b:else/>
<style>#fullpost {display:none;}</style>
<p><data:post.body/></p>
<span id='showlink'>
<p><a expr:onclick='"javascript:showFull(\"post-" + data:post.id + "\");"' href='javascript:void(0);'> Show Full Post...</a></p>
</span>
<span id='hidelink' style='display:none'>
<p><a expr:onclick='"javascript:hideFull(\"post-" + data:post.id + "\");"' href='javascript:void(0);'> Hide Full Post...</a></p>
</span>
<script type='text/javascript'>checkFull("post-" + "<data:post.id/>")</script>
</b:if>

5. For the next step find </head> and before it paste the next script:

<script type='text/javascript'>
//<![CDATA[
var fade = false;
function showFull(id) {
 var post = document.getElementById(id);
        var spans = post.getElementsByTagName('span');
        for (var i = 0; i < spans.length; i++) {
             if (spans[i].id == "fullpost") {
                 if (fade) {
                    spans[i].style.background = peekaboo_bgcolor;
                    Effect.Appear(spans[i]);
                } else spans[i].style.display = 'inline';
             }
             if (spans[i].id == "showlink")
                 spans[i].style.display = 'none';
             if (spans[i].id == "hidelink")
                 spans[i].style.display = 'inline';
        }
}


function hideFull(id) {
 var post = document.getElementById(id);
        var spans = post.getElementsByTagName('span');
        for (var i = 0; i < spans.length; i++) {
             if (spans[i].id == "fullpost") {
                 if (fade) {
                   spans[i].style.background = peekaboo_bgcolor;
                   Effect.Fade(spans[i]);
                 } else spans[i].style.display = 'none';
             }
             if (spans[i].id == "showlink")
                 spans[i].style.display = 'inline';
             if (spans[i].id == "hidelink")
                 spans[i].style.display = 'none';
        }
        post.scrollIntoView(true);
}

function checkFull(id) {
 var post = document.getElementById(id);
        var spans = post.getElementsByTagName('span');
        var found = 0;
        for (var i = 0; i < spans.length; i++) {
             if (spans[i].id == "fullpost") {
                 spans[i].style.display = 'none';
                 found = 1;
             }
             if ((spans[i].id == "showlink") && (found == 0))
                 spans[i].style.display = 'none';
        }
}


//]]>
</script>

6. Save your template.
7. Go to menu "Setting --> Formatting"
On the bottom page you will see a box beside text "Post Template". Copy the code below and paste in to the box.

<span id="fullpost">


</span>

Save your settings.

8. If you want to post an article place your part article above this code <span id="fullpost">. Then the rest or full post put between this code <span id="fullpost"> and this </span>.

IMPORTANT: When you write the post( article) use the HTML editor  for the step 8.
DEMO