In a large bowl, beat the dairy free butter, powdered sugar and vanilla for 2 minutes until smooth and creamy. Use firm dairy free butter sticks rather than a tub spread for the best buttery texture.
Add the flour, cornstarch and salt and mix on low speed until a soft, uniform dough forms.
Divide the dough in half and shape each piece into a log about 1 1/2 inches wide on a sheet of plastic wrap. Wrap tightly and chill for at least 2 hours, until very firm.
Preheat the oven to 350°F and line two baking sheets with parchment paper. Roll each chilled log in the coarse sugar, pressing gently so it sticks.Slice the logs into 1/3 inch thick rounds and place them 1 inch apart on the baking sheets.
Bake for 12-14 minutes, until the edges are just barely golden. Cool on the sheet for 5 minutes, then move to a wire rack to cool completely, as the cookies firm up while they cool.
Microwave the dark chocolate chips with the coconut oil in 30-second bursts, stirring between each, until completely smooth.
Dip half of each cookie into the melted chocolate, let the excess drip off and set it on parchment paper. Sprinkle with a pinch of flaky sea salt and chill for 15 minutes until the chocolate is set.