Hamburger Button

Displays a hamburger menu which transitions to a cross on hover.

<div class="hamburger-menu">
  <div class="bar top"></div>
  <div class="bar middle"></div>
  <div class="bar bottom"></div>
</div>

.hamburger-menu {
  display: flex;
  flex-direction: column;
  flex-wrap: wrap;
  justify-content: space-between;
  height: 2.5rem;
  width: 2.5rem;
  cursor: pointer;
}

.hamburger-menu .bar {
  height: 5px;
  background: black;
  border-radius: 5px;
  margin: 3px 0px;
  transform-origin: left;
  transition: all 0.5s;
}

.hamburger-menu:hover .top {
  transform: rotate(45deg);
}

.hamburger-menu:hover .middle {
  opacity: 0;
}

.hamburger-menu:hover .bottom {
  transform: rotate(-45deg);
}

Explanation

Use a .hamburger-menu container div which contains the top, bottom, and middle bars.
The container is set to be a flex container (display: flex) with flex-direction to be column and flex-wrap to be wrap (alternatively, you can set both properties by a shorthand flex-flow: column wrap).
Add distance between the bars using justify-content: space-between.
The animation has 3 parts: top and bottom bars transforming to 45 degree angles (rotate(45deg)), and the middle bar fading away by setting opacity: 0.
The transform-origin is set to left so the bars rotate around the left point.
Set transition all 0.5s so that both transform and opacity properties are animated for half a second.