Put the sugar into a heavy-bottomed saucepan and place over medium heat. Leave the sugar over the heat for about 10 to 15 minutes, shaking the pan every few minutes. The sugar should still be white and granular, but will smell of caramel.
Put the butter into the saucepan a little at a time; the butter should begin to melt instantly once it hits the pan. Whisk in all of the butter. The mixture should be smooth and glossy. Leave on very low heat until ready to use.
Remove the chute and put the kernels into the popping chamber. Replace chute and insert butter warming cup.
Place a large bowl underneath the chute and turn unit on.
Once the last kernel has popped, turn unit off. Pour the sugar mixture over the popped corn and sprinkle in the salt; mix to fully coat. Serve immediately.