How to Make It
Preheat the oven to 350°. Line 2 large baking sheets with parchment paper. In a medium bowl, whisk the flour, cornmeal, milk powder, baking powder, baking soda, rosemary and 1/2 teaspoon of the salt.
In a food processor, pulse the corn with the lemon zest until very finely chopped.
In the bowl of a standing mixer fitted with the paddle, beat the butter with 3/4 cup of the sugar at medium-high speed for 2 minutes, until smooth. Beat in the egg until incorporated. At low speed, beat in the chopped corn mixture, then add the molasses, lemon juice and vanilla. Beat at high speed until pale and fluffy, about 5 minutes. At low speed, beat in the dry ingredients in 3 additions.
Working in 2 batches, scoop 11/4-inch mounds of dough onto the prepared baking sheets, about 2 inches apart. With damp fingers, flatten each mound slightly. In a small bowl, mix the remaining 3 tablespoons of sugar and 1/2 teaspoon of salt and sprinkle half of the mixture on the cookies. Bake in the upper and lower thirds of the oven for 12 to 15 minutes, until golden brown; shift the pans from top to bottom and front to back halfway through. Slide the parchment paper from the baking sheets onto racks. Repeat to bake the remaining dough.
For a slice-and-bake version of these cookies, refrigerate the dough for 30 minutes, until no longer sticky. Form it into 2 logs, 2 1/2 inches in diameter. Wrap tightly in plastic and freeze until firm or for up to 1 month. Slice the logs 1/4 inch thick and proceed as above.