Rub the cold butter into the gluten-free flour, salt, and sugar in a bowl using your fingertips.

Add the water in 3 additions, mixing until the dough comes together into a ball.

Wrap the dough in plastic wrap.

Refrigerate for at least 30 minutes.

Roll out the dough between two sheets of parchment paper to keep it from sticking to the work surface.

Roll it to an even thickness of about 3-4 mm.

Cut the dough to fit the tartlet molds.

Press the dough into the tartlet molds.

Freeze for at least 30 minutes.

Preheat the oven to 180 °C.

Bake for 18 to 20 minutes.

Let the tart shells cool completely at room temperature before filling.

Bring the cream to a boil in a saucepan.

Pour the hot cream over the chocolate in a bowl.

Blend with an immersion blender until smooth and glossy.

If you don't have an immersion blender, pour the hot cream over the chocolate in 3 additions.

Stir constantly until smooth and glossy.

Fill the tart shells with the warm chocolate ganache.

Gently tap a few times to level the filling.

Sprinkle cocoa nibs over the top.

Chill for 10 minutes, or until set, before serving.