What is "to mill chocolate"

Mill \Mill\ (m[i^]l), v. t. [imp. & p. p. Milled (m[i^]ld); p. pr. & vb. n. Milling.] [See Mill, n., and cf. Muller.]

1. To reduce to fine particles, or to small pieces, in a mill; to grind; to comminute.

2. To shape, finish, or transform by passing through a machine; specifically, to shape or dress, as metal, by means of a rotary cutter.

3. To make a raised border around the edges of, or to cut fine grooves or indentations across the edges of, as of a coin, or a screw head; also, to stamp in a coining press; to coin.

4. To pass through a fulling mill; to full, as cloth.

5. To beat with the fists. [Cant]
   --Thackeray.

6. To roll into bars, as steel.

   To mill chocolate, to make it frothy, as by churning.