Preheat the oven to 350°F (175°C). Grease and flour individual cake molds or a baking dish.
In a heatproof bowl, melt the dark chocolate using a double boiler or microwave. Slowly add the vegetable oil to the melted chocolate, along with the milk and egg yolks. Allow it to cool slightly.
In a large mixing bowl, beat the eggs white with sugar until pale and fluffy.
Gradually incorporate the melted chocolate into the egg mixture, stirring until well combined.
In a separate bowl, sift together the all-purpose flour and baking powder. Add the instant coffee powder and mix well.
Alternately add the flour mixture and milk to the chocolate batter, beginning and ending with the flour mixture. Stir until just combined; do not overmix.
Pour the batter into the prepared cake molds or baking dish, filling each mold about three-quarters full.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center of the cakes comes out clean.
While the cakes are baking, prepare the "camisa" (cream). In a saucepan, heat the milk over medium heat until it begins to simmer. Remove from heat and stir in the vanilla extract (or grated sarrapia zest).
In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until smooth and creamy.
Slowly pour the hot milk mixture into the egg yolk mixture, whisking constantly to prevent curdling.
Return the mixture to the saucepan and cook over low heat, stirring constantly, until it thickens to a creamy consistency.
Remove from heat and let the "camisa" cool to room temperature.
Once the cakes are baked and cooled, pour the prepared "camisa" over the top of each cake, covering it completely.
Allow the cakes to set in the refrigerator for at least 1 hour before serving.
Serve the "Negro en Camisa" chilled, garnished with chocolate shavings or a sprinkle of cocoa powder, if desired.