1) To make the filling: Scoop small balls of Nutella (chestnut-sized, about 1" diameter) onto a parchment-lined baking sheet. A teaspoon cookie scoop, filled level, is the perfect tool for this job; your goal is about 2 level measuring teaspoons of Nutella. You should have about 12 small balls.
2) Place the baking sheet into the freezer (uncovered is fine), and freeze until the balls are completely frozen, about 3 hours; or up to overnight.
3) Preheat the oven to 350°F. Lightly grease a baking sheet, or line it with parchment.
4) To make the dough: Mix together all of the dough ingredients; the mixture will be cohesive, fairly soft, but not sticky; think modeling clay.
5) Scoop out heaping tablespoonfuls of the dough; a slightly heaped tablespoon cookie scoop works well here.
6) Flatten a ball of dough. Place one of the frozen Nutella balls in the center. Wrap the dough around the Nutella like a dumpling, enclosing it completely. Roll the ball of dough between your palms to seal any cracks and round it out. Repeat with the remaining dough and frozen Nutella balls.
7) Place the cookies on the prepared baking sheet; they won't spread much, so should all fit on one sheet. Sprinkle very lightly with a bit of coarse sea salt.
8) Bake the cookies for 8 to 10 minutes; when done, they will have lost much of their shine, and you may see a very faint lightening of color around the bottom third of each cookie.
9) Remove the cookies from the oven; serve warm, or at room temperature. For the full melting-center, lava-like effect, serve warm; if they're at room temperature, the centers will be solid. Reheat very briefly in the microwave to liquefy the centers, if desired.
Yield: 12 rich cookies.