Make the ganache: Place the chocolate and butter in a large heatproof bowl. Bring the cream just to a simmer, then pour it over the chocolate mixture. Let stand for 1 minute, then blend with an immersion blender until smooth and glossy. Press plastic wrap directly onto the surface and refrigerate until thickened and spreadable, at least 4 hours.

Make the cake: In a large bowl, whisk the eggs and sugar until slightly thickened and pale. Whisk in the oil until fully incorporated. Sift together the flour, cocoa powder and baking soda, then fold into the egg mixture. Stir in the milk until combined. Add the hot water and whisk just until the batter is smooth and fluid.

Divide the batter evenly between the prepared pans. Bake until the tops spring back lightly and a toothpick inserted in the center comes out with moist crumbs, 35 to 40 minutes. Cool in the pans for 15 minutes, then turn out onto racks and cool completely. Repeat the recipe to produce 4 layers total.

Make the syrup: In a small saucepan, combine the sugar, water and cocoa powder. Bring to a boil over medium heat, stirring until dissolved. Remove from heat and cool completely.

If necessary, level the cakes with a serrated knife. Place one layer on a serving plate and brush generously with syrup. Spread about one-third of the ganache evenly over the top, taking it to the edges. Top with a second layer and repeat. Continue with the third layer. Place the final layer on top and brush lightly with syrup.

Frost the top and sides with the remaining ganache, smoothing with an offset spatula. Refrigerate until set, at least 1 hour. Slice with a warm knife for clean edges.