To soften the butter, place it in a medium microwavable bowl and microwave in 10 second increments until softened but not melted. Add the maple syrup and use an electric mixer to beat until combined and creamy (it won’t get fluffy). Add the yolk and vanilla and beat again until combined.
Add the hazelnut meal, flour, and salt and use a rubber spatula to mix until the dough comes together. You should have a soft uniform dough.
Cover the dough with plastic wrap, placing the plastic wrap directly on top of the dough and pressing it to cover. Cover and chill in the fridge for at least an hour or ideally for 24-48 hours.
Once the dough has chilled, preheat the oven to 350F and line a baking sheet with parchment paper.
If the dough is too hard when you take it out of the fridge, let it sit out for 5-10 minutes until it is scoopable. Use a tablespoon to scoop the dough onto the baking sheet, then roll them into balls, Roll the balls into leftover toasted hazelnut meal to coat.
Arrange the balls on the baking sheet. Use a round ½ teaspoon measure (or your finger if you don’t have one) to press an indent into the center of each cookie. If the cookie cracks a little on the edges, just gently press it back together.
Bake 12-15 minutes or until the cookies are slightly firm on the edges. If the indents puffed up, gently tap the ½-teaspoon measure in the center. Let the cookies cool completely before filling or eating. They are very delicate when warm.
To make the chocolate ganache: in a microwavable bowl, add the chocolate. Melt in the microwave in 20-30 second intervals, stirring after each time until just melted. Whisk in the coconut milk until totally smooth.
Spoon the chocolate mixture to fill the center well of each cookie. To decorate, place a whole roasted hazelnut in the center and gently press it down. Finish with a sprinkle of flaky salt.
Let the cookies/chocolate cool completely! You want that chocolate to solidify into a fudgy texture.
Store cookies in an airtight container at room temperature for 3–4 days. For longer storage, refrigerate up to 1 week or freeze up to 2 months.
If frozen, thaw at room temperature and let the ganache soften before serving.