Heat the oven to 350°F. Cover the pitted dates with the ¼ cup warm water in a small bowl and let them soften for 10 to 15 minutes. In a second small bowl, stir the chia seeds into the 3 tablespoons water and set it aside to gel.
Spread the hazelnuts on a baking sheet and roast for 10 minutes, until they smell toasty and look golden. Let them cool for a few minutes, then rub off any loose skins in a kitchen towel if you like.
Pick out 16 whole hazelnuts for the centers. Finely chop 2 tablespoons of the others for the coating.
Pulse the rest of the hazelnuts in a food processor into small crumbs. Add the dates along with their soaking water, then the almond meal, coconut, chia gel, and cacao powder. Blend until everything clumps into a sticky dough that holds together when pinched.
Scoop about 1½ tablespoons of dough, push a whole hazelnut into the middle, and roll it into a smooth ball. Repeat with the rest of the dough to make 16. Freeze the balls for 10 minutes.
Melt the dark chocolate with the coconut oil, if using, in a small saucepan over very low heat or a double boiler, or in the microwave in 30-second bursts, stirring until smooth. Stir in the 2 tablespoons chopped hazelnuts.
Roll each ball in the chocolate until it's covered, then lift it out with a fork or tongs and let the extra drip back into the pot. Set it on a parchment-lined plate.
Refrigerate for 30 minutes, until the chocolate shell is firm. For an icy, firmer bite, set them in the freezer instead.