I was following a simple butter tablet recipe. After 20 minutes of stirring and boiling the condensed milk, sugar, butter and milk mixture, I removed the pot from the heat, and started to beat. This was looking good, and getting sticky, and a darker brown, when suddenly the thickening mixture started to rise. I was forced to go to the sink to handle the problem, and the now quite porous mixture poured over the edges of the pot.
This happened twice. I blamed the first failure on using an electric hand mixture for the beating phase. I'm using a copper bottom pot. What went wrong?