In addition to Andrew's comment above about using the outlined star icon when the button isn't favorited, we should apply the icons internal to the Button component when it's a favorite button since we also need to wrap these star icons in additional class wrappers and both icons should be rendered in the DOM at the same time (rather than dynamically rendering the correct icon, as these additional class wrappers will determine which one is hidden or not).

Here's the Core markup for the icons:

vs the markup in the PR preview:

Though @andrew-ronaldson would we want to allow a consumer to customize the favorite icon(s) at all? If we always have both icons rendered we'd most likely need to add some other props (favoriteIcon and favoritedIcon or something) since we need to place the correct icons in the correct icon wrapper. Dynamically rendering the correct star icon might be a little cleaner, since then you only need to pass in the existing icon prop, and the animation still seems to work, but it makes some of the Core styling unused in React/creates a mismatch in markup between Core and React.

Figure I'd ask this now because depending on the answer (and the approach if the answer is "yes"), might be less of a headache to deal with it now than try to finagle something without causing a breaking change to the markup later.