In the bowl of your stand mixer, cream together the butter and sugar until fluffy.
Add the salt and then the flour slowly (so as not to create an poof of flour), until it is incorporated.
Chop the chocolate bar into small pieces, and fold the chocolate into the dough.
Divide the dough into three even lumps, and then roll the lumps out into logs about 2" in diameter. Wrap the logs snugly in wax paper or saran wrap, then place in the fridge to set.
Preheat oven to 350 degrees.
Slice the logs into 1/4" to 1/2" slices, and place on a cookie sheet. Sprinkle with a little extra grated chocolate and sea salt if desired.
Bake for around 9 minutes, until the edges are just beginning to be golden.
Remove and allow to cool on a wire rack.