In the Web.config file located inside of your Views folder, place a namespace inclusion.
<pages>
<namespaces>
<add namespace="TestingGround01.App_Code">
</namespaces>
</pages>
To clarify, this is useful for EXTERNAL references. In your case, if you simply stick a
@using TestingGround01.App_Code
at the top of your _layout.cshmtl file, it should resolve the reference just fine.
Then, you can instantiate your class inside your _layout.
@{ var s = new SessionHandler(); }