In a large bowl, beat the butter and brown sugar until pale and fluffy; beat in the molasses, milk and vanilla. Add the dry ingredients and beat on low speed or stir by hand until you have a soft dough. Wrap in plastic or waxed paper and chill for at least 30 minutes.
Divide the dough into four pieces and roll each piece out between two sheets of parchment paper to about 1/8-inch thin. Move the rectangle to a baking sheet, peel off the top layer of parchment, cut into squares or rectangles and freeze until firm. Meanwhile, preheat the oven to 350F.
Remove from freezer, prick all over with a fork and bake for 10 minutes, until golden. If you like, separate the crackers and bake for a few more minutes.