Begin this bubble bar recipe by sifting all of your dry ingredients together to be sure there are no lumps. Add the melted butter and oil and stir well. You may want to put gloves on to mix it well. Add the soap in a little at a time, mixing well after each addition. Drip the color and mix well, or divide the mixture into two or three bowls and color each one separately. Add the essential oil and mix it in. You should have a dough-like mixture. If it's too dry, add a bit more liquid soap. You can also add a bit more cornstarch or arrowroot powder if it's too wet.
Turn out onto a powdered surface and knead like bread dough. When it gets firm, you're ready for the next step.
Shape the bubble bar recipe bars any way you like. You can layer colors, roll them into a log, and cut them to the desired size. It can also be shaped into balls. Try adding bits of each color to the balls as you are shaping them for a fun effect. I like to make mine into "truffles."
Finally, let your DIY bubble bar dry out for 2-3 days. Once they are thoroughly dry, place them in a jar with a tight-fitting lid. These will keep for a few months unless you use them up before then!