Preheat the oven to 170°C / 340°F and line the 15 cm square cake pan with parchment paper.

Sift the all-purpose flour, cocoa powder, baking powder, baking soda, and salt into a mixing bowl.

Melt 60 g dark chocolate with the vegetable oil over a bain-marie until smooth.

Mix the vanilla extract into 60 ml whole milk until evenly combined.

Beat the 2 room-temperature eggs, granulated sugar, and cake emulsifier until pale, thick, and ribbon-like.

Fold half of the sifted dry ingredients into the egg mixture just until no dry flour remains.

Fold in the melted chocolate-oil mixture and the vanilla milk until evenly combined.

Fold in the remaining dry ingredients only until the batter is smooth; do not overmix.

Pour the batter into the prepared cake pan and level the surface.

Bake the cake for about 30 to 35 minutes until a skewer inserted in the center comes out clean.

Cool the baked cake in its pan for 10 minutes, then transfer it to a wire rack until completely cold.

Warm 125 ml whole milk in a saucepan until steaming but not boiling.

Pour the warm milk over 250 g dark chocolate and the butter, then stir slowly until smooth.

Return the ganache to a bain-marie and stir gently if any chocolate or butter lumps remain.

Let the ganache cool completely until thickened but spreadable.

Spread the cooled ganache evenly over the completely cooled cake.

Refrigerate the cake for at least 60 minutes until the ganache is set.

Trim the cake edges if desired and cut the cake into 9 equal squares before serving.