Measure out 1/4 cup coconut oil, 3 tbsp plant-based milk, and 3/4 tsp salt into separate containers. Have them waiting near the stove.
Start with a very clean pan and pour in 1 cup sugar, 3 tbsp water, and 1/8 tsp cream of tartar.
Turn on the heat to medium and let it start to boil. (Don’t stir, just swish it around a little if needed).
Allow the sauce to boil for about 5 minutes and then start watching it very closely.
After about 6-7 minutes, the bubbles will get smaller and it will start to get a darker color and smell slightly of caramel.
When it starts to caramelize, remove it from the burner, and immediately pour in the 1/4 cup coconut oil. (At this point you can give it a stir with a long-handled whisk).
Pour in the 3 tbsp plant milk and continue to stir with the whisk for about 30 seconds. (It will bubble up and spatter when you add the milk, but then turn to caramel after a few seconds of stirring).
Once the bubbling from the milk calms down, add the 3/4 tsp salt and stir for a few more seconds. (or leave out the salt for regular caramel).
Pour the hot caramel onto the shortbread layer and smooth it out if needed. Put in the fridge to cool.
Make the chocolate layer:
Melt 1 1/2 cups of dairy-free chocolate chips with 3 tbsp of vegan butter in the microwave or in a double boiler. (You can not melt chocolate chips directly on the stovetop). (I find that the easiest way is in a pyrex measuring container and putting it in the microwave for 1 minute 15 seconds.)
Stir the melted chocolate and butter with a fork until they are completely melted and blended together then pour it over your caramel layer and smooth it out with a spatula.
Cut into bars:
Refrigerate your vegan millionaire shortbread for 2 hours until the caramel and chocolate are firm, then pull them out of the pan holding the parchment paper and cut them into bars. (You can sprinkle the tops with coarse salt if you wish.)
Notes
Make sure to get the shortbread layer as smooth and even as possible. If you have areas where the shortbread is thicker or thinner, your caramel will not be a uniform thickness.
Be sure to follow the steps for the caramel exactly. Caramel is a candy-making process which is an exact science. Slight changes to the recipe will cause your caramel to fail. (Even differences in humidity will make the caramel come out slightly different each time you make it.) If you want more detailed help with the caramel recipe, be sure to check out my vegan salted caramel recipe.
Don't overheat the chocolate chips! Heat them until they just start to melt. The process of stirring them will cause them to melt the rest of the way.
Use a large sharp knife to cut them into squares and push down firmly to get a clean cut.
For perfect squares first cut 1/4 inch off of each side to make perfect edges. Then cut the large square in half in both directions. Cut those squares in half in both directions too and you should have near-perfect squares.