In a large bowl, whisk together the flour, sugar, and salt, and set aside. In a small saucepan, warm the milk over low heat. Add the warmed milk and butter to the flour mixture, followed by the eggs and the rum. Use an immersion blender to mix the batter until thoroughly combined. Cover the bowl with plastic wrap and let rest at room temperature for at least 1 hour, or in the refrigerator overnight.
To make the crepes, spray an 8-inch saute pan with grapeseed oil spray, wipe it clean, and turn the heat to high. (You'll know the pan is ready when it just begins to smoke.) Using a 2-ounce ladle, ladle out crepe batter into the hot pan and tilt the pan to spread the batter into an even circle. After 45 seconds, when the top is dry, flip the crepe over and cook on the other side for 20 seconds, until it just starts to brown at the edges. Fold the crepe in half and transfer it to a sheet pan lined with foil. Repeat this process with the remaining crepe batter.
Place one crepe on a plate and top with Hot Chocolate Sauce. Fold the sides over and repeat with another crepe on the same plate (you will have two crepes for each serving.) Repeat with the remaining crepes and serve.
Hot Chocolate Sauce:
Place the chocolate in a large bowl and set aside. Combine the water, sugar, cocoa powder, and cream in a medium saucepan and bring to a simmer over medium heat. Pour the hot cream mixture over the chocolate, setting the saucepan aside, and whisk until smooth. Pour the chocolate sauce into a sieve set over the reserved saucepan and keep warm until ready to use.