Line a 4x7-inch (10x18 cm) baking dish with parchment paper and set aside.

Add the shortbread ingredients to a bowl. Add the oat flour, maple syrup, coconut oil, and vanilla extract to a bowl. Use your hands to mix the ingredients until a soft dough forms (similar to cookie dough).

Transfer the dough to the baking dish and use your hands to press it into an even layer. Top with a layer of parchment paper, then press it down firmly with the bottom of a glass. Set aside.

Add the pistachios, maple syrup, and salt to a food processor. Process for 30-40 seconds, or until it has the texture of almond meal. The mixture should be slightly sticky and hold together if you try to shape it into a ball.

Transfer the pistachio mixture into the baking dish, on top of the oat shortbread. Press it firmly into an even layer. Again, top with a layer of parchment paper and use a glass to press it down firmly into an even layer.

Place the chocolate chips (or chopped dark chocolate) in a small bowl. Heat the milk in a small saucepan until it almost comes to a boil. Immediately pour the hot milk into the bowl and let it sit for 1 minute. Next, whisk until the chocolate is melted and fully combined with the milk.

Pour the ganache over the pistachio layer and tilt the dish to spread it into an even layer.

Refrigerate for at least 4 hours before cutting into 12 squares.