How to replace the Sign in and Sign out with an icon?

Question

I'm trying to replace the "Sign in" Link in header.links with an icon. Search stack exchange I was only able to add

 <referenceBlock name="header.links">
        <block class="Magento\Customer\Block\Account\Link" name="my-account-link">
            <arguments>
                <argument name="label" xsi:type="string" translate="true">My Account</argument>
                <argument name="sortOrder" xsi:type="number">110</argument>
                <argument name="class" xsi:type="string" translate="true">account-link</argument>
            </arguments>
        </block>
    </referenceBlock>

this to default.xml

That's all nice and good but it only adds a new link "My Account" which I could replace with an icon with CSS. I can also hide the "Sign in" with CSS but if signed in, then there is no Sign Out since the "My Account" link will only forward to the account.