How can I have Brand and Navbar on separate lines?

Pretty simple, but I'm fairly amateur with Bootstrap.

I understand how to make the navigation bar with an image as the brand and the nav-items. Without hacking it, is there a proper way to have the brand positioned above the expanded navigation?

Basically looking for this;

EXPANDED
-----------------------------------------
|                 Logo                  |
-----------------------------------------
|         Home   About   Contact        |
-----------------------------------------

COLLAPSED
-----------------
| Logo        ≡ |
-----------------

<nav class="mainNav navbar navbar-expand-md justify-content-center">
    <a class="navbar-brand" href="#">
        <img src="img/logo.png" alt="Logo" height="80px" width="auto">
    </a>
    <ul class="navbar-nav">
        <li class="nav-item">
            <a class="nav-link" href="#">Home</a>
        </li>
        <li class="nav-item">
            <a class="nav-link" href="#">About</a>
        </li>
        <li class="nav-item">
            <a class="nav-link" href="#">Contact</a>
        </li>
    </ul>
</nav>

Use flex-column to make the navbar items stack in 2 rows (lines).

group the logo & toggler together in 1 flexbox div (d-flex w-100)
use mx-md-auto (auto-margins) to center the logo on larger widths
use text-center to center items in the navbar-nav

https://www.codeply.com/go/W9HQcd3Pyw

<nav class="mainNav navbar navbar-expand-md navbar-light flex-column">
    <div class="w-100 d-flex">
        <a class="navbar-brand mx-md-auto" href="#">
            <img src="img/logo.png" alt="Logo" height="80px" width="auto">
        </a>
        <button class="navbar-toggler ml-auto" type="button" data-toggle="collapse" data-target=".navbar-collapse">
            <span class="navbar-toggler-icon"></span>
        </button>
    </div>
    <div class="collapse navbar-collapse">
        <ul class="navbar-nav text-center">
            <li class="nav-item">
                <a class="nav-link" href="#">Home</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">About</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#">Contact</a>
            </li>
        </ul>
    </div>
</nav>

Read more about the Navbar and Utility classes in the Bootstrap 4 docs.

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