Preheat the oven to 350°F (175°C), conventional setting. Line standard muffin tins with cupcake liners.

In a mixer bowl, whisk together the flour, brown sugar, granulated sugar, cocoa, baking powder, baking soda, and salt.

Add the oil, milk, egg, vanilla, water, and sour cream. Mix on low until combined, scraping the bowl. Mix on medium for about 30 seconds, just until evenly combined.

Fill each liner about halfway. Bake for 14–16 minutes, until the centers are set and a toothpick inserted into the center comes out without wet batter. Transfer the cupcakes to a wire rack once they are firm enough to handle. Cool completely.

While the cupcakes bake, combine the evaporated milk, egg yolks, brown sugar, and butter in a saucepan. Heat over low, stirring frequently, until gently simmering. Continue cooking and stirring for about 2–3 minutes, until thickened.

Remove the saucepan from the heat. Stir in the vanilla, coconut, and chopped pecans. Set aside to cool completely.

Beat the softened butter and cream cheese until smooth. Add the powdered sugar and cocoa. Mix on low until combined, then beat on medium for 1 minute.

Mix in the melted chocolate once it has cooled to room temperature. If the frosting is dry and crumbly, add a little of the optional milk, using no more than 1 tablespoon. Otherwise, leave the milk out. Beat on medium for another minute, scraping the bowl, until smooth and fluffy.

Transfer the frosting to a piping bag fitted with a large star tip. Pipe a ring around the outside edge of each completely cooled cupcake, leaving the center uncovered. Do not remove any cake.

Spoon a small dollop of cooled coconut-pecan topping into each frosting ring. Gently settle it into the space, then distribute any remaining topping evenly.

Keep the decorated cupcakes in an airtight container in the refrigerator for up to 3 days.