Heat the oven to 350°F (175°C). Lightly grease a 9-by-13-by-2-inch metal baking pan with the 1 teaspoon oil and line it with parchment paper.

Whisk the sugar, flour, natural cocoa, baking powder, baking soda and salt together in a large bowl. Break up visible cocoa lumps so the mixture looks evenly brown.

Add the eggs, milk, 1/2 cup oil and vanilla. Beat with a hand mixer on medium speed for 2 minutes, scraping down the bowl once. Carefully stir in the boiling water by hand until smooth. The batter will be thin; do not add more flour.

Pour the batter into the prepared pan. Bake on the middle rack for 35–40 minutes, until a toothpick inserted in the center comes out without wet batter and the top springs back gently. Ovens vary; continue in short intervals if needed.

Set the pan on a wire rack and let the cake cool completely before frosting, about 60–90 minutes. Leave it in the pan for easy serving.

Put the chopped chocolate in a heatproof bowl. Heat the cream in a small saucepan just until steaming with tiny bubbles at the edge; do not boil hard. Pour over the chocolate, leave for 5 minutes, then stir from the center outward until smooth. Let it thicken to a spreadable consistency, about 10–20 minutes. If small chocolate pieces remain, warm the bowl very briefly over barely simmering water and stir.

Spread all the ganache in a thin, even layer over the cooled cake. Let it set for about 15 minutes, or chill briefly. Cut into sixteen squares. Refrigerate covered once finished; bring portions out briefly before serving.