Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper.

Sift flour, cocoa powder, baking soda and baking powder into a medium bowl. Add salt and whisk to blend well.

Whisk together eggs with sugar in a medium bowl until pale and thick. Mix in vanilla. Stir in melted butter and oil. Gradually whisk in half of the dry ingredients until just combined. Whisk in buttermilk. Mix in remaining dry ingredients until evenly incorporated. Do not over-mix.

Spread the batter into prepared pan and bake for 25-30 minutes until it springs back when pressed gently and a toothpick inserted in the center comes out clean. Transfer to a wire rack to cool completely before glazing.

For the White Chocolate Tahini Glaze, combine chopped white chocolate with cream in a small saucepan over very low heat. Stir until melted and smooth. Remove from heat and stir in tahini. Let stand until thick and spreadable.

For the Milk Chocolate Glaze, combine chopped milk chocolate with cream and a pinch of salt in a small saucepan over very low heat. Stir until melted and smooth, then let stand until it cools down a bit but is still runny. Pour over cooled cake and spread it out. Spoon the white chocolate ganache randomly on top and swirl it through with a skewer. Cover and refrigerate until set, then serve.