Preheat the oven to 375°F. In a large bowl, whisk the flour, baking mix, cinnamon and salt together until combined. In another small bowl, stir the evaporated milk, oil and brown sugar together until dissolved and pour into the flour. With a large rubber spatula, mix the wet and dry ingredients together until a soft dough forms. Using your hands, knead the dough in the bowl until smooth, about 2 minutes. Wrap the dough tightly in plastic wrap and let stand at room temperature for 10 minutes.

On a lightly floured work surface, roll the dough into a large circle, about 1/8-inch thick. Using a round cutter or glass the same diameter as the top of the cup in a muffin tin, cut 12 circles from the dough, re-rolling the scraps if necessary. Using your fingers, press the circles evenly into the bases of a 12-portion non-stick muffin tin, pressing firmly into the corners. (The dough will come about 1/4 way up the sides of the cup.) Lay 2 pieces of chocolate into each tart shell in a single layer, followed by a marshmallow half. Bake until the crust is golden brown, and marshmallow is puffed and lightly golden, 10 to 12 minutes. At this point you can also broil them briefly if you like the marshmallow on your S’mores deep golden brown.

Remove from the oven and let cool 5 minutes in the pan. Transfer the tarts to a serving platter and dust them with cocoa powder. Serve warm.