Weigh your bath butter. Measure 91 g of your foaming bath butter into a mixing bowl. If it's been sitting a while, you may notice it's settled or firmed up a bit — that's completely normal.
Give it a good whip first. Before adding anything else, run your hand mixer through the base until it's light and fluffy again — you'll feel it go from dense and slightly grainy back to soft, billowy peaks. This step matters more than it seems — starting from a properly whipped base is what gives the finished shaving cream its silky, cloud-like texture.
Add the fragrance oil and castor oil together, then mix well — this is a good spot to really hit it with the hand mixer, letting the oils fully work into the whip until everything looks glossy and uniform.
Add the bentonite clay and mix again until it's fully incorporated and the texture turns a soft, even stone-grey.
Add the titanium dioxide a small pinch at a time, mixing well between additions, until the color brightens to whatever shade of white or ivory you love most.
Jar it up. Transfer to a clean, sanitized container and label it clearly — this is a shaving cream now, distinct from your original bath butter.