Preheat the oven to 350°F (180°C, or gas mark 4).

Line 2 baking sheets with parchment paper or silicone baking mats.

In a large mixing bowl, use an electric mixer to cream together the nondairy butter and powdered sugar until light and fluffy, about 2–3 minutes.

Stir in the sea salt, arrowroot powder, vanilla extract, flour, and baking soda until well combined.

Shape the dough into a ball and divide it in half.

Set one half aside.

Using an electric mixer, beat the cocoa powder and nondairy milk into one half of the dough until well combined.

If the dough feels too dry, add a bit more milk.

Flatten both the vanilla and chocolate doughs into separate rectangles of equal size, about ¾ inch (2 cm) thick.

Cut each rectangle in half.

Stack them alternately (vanilla-chocolate-vanilla-chocolate), gently pressing them together to form a rectangular roll.

Wrap the dough in plastic wrap and refrigerate for 1 hour.

Remove the dough and slice into 15 equal pieces.

Arrange the slices on the prepared baking sheets, leaving 1 inch (2.

5 cm) between cookies.

Bake for 15 minutes, or until the edges are slightly golden and the tops are almost firm.

The cookies will firm up as they cool.

Let the cookies cool for a few minutes on the baking sheet before transferring them to a wire rack to cool completely.