Step 1

To make the cookie base: Combine the butter and powdered sugar with a stand or hand mixer until fluffy. Add the egg yolk, vanilla extract and sea salt until combined. Add the flour and 1-2 Tbsp water, as needed until a crumbly dough forms.

Step 2

Shape into a disc, cover and refrigerate for at least 20 minutes, or overnight.

Step 3

Preheat the oven to 350°F. Line a baking tray with parchment paper. On a lightly floured surface, roll the dough to ¼ inch thickness. Cut 2 ½ inch circles out of the dough, re-roll the scraps to cut into circles. Transfer to the baking tray and refrigerate for 10 minutes.

Step 4

Bake for 11-13 minutes, or until golden.

Step 5

Transfer to a cooling rack until they cool to room temperature.

Step 6

To make the marshmallow cream: In a small saucepan, heat the sugar and water to 240°F over medium heat. Once the sugar mixture reaches 200°F, begin whipping the egg whites and cream of tartar in a stand mixer to stiff peaks.

Step 7

Once the sugar mixture reaches 240°F, pour the sugar syrup into the whipping egg whites, beating on high for 5-7 minutes, or until stable and shiny.

Step 8

Transfer to a piping bag fitted with a large, round tip. Holding the piping bag vertically, pipe a tall mound on each cookie.

Step 9

Transfer to the fridge until set.

Step 10

To make the chocolate shell: Melt the chocolate in a double boiler (or in the microwave in 30-second increments). Add the coconut oil, mixing to combine. It should be the consistency of heavy cream.

Step 11

Spoon the chocolate coating over the marshmallow. Return to the fridge to set.