Heat oven to 350 degrees. Grease sides and bottom of an 11 x 17 rimmed baking pan and then line the bottom with parchment.

Mix cocoa powder, oil, water, and 1/2 cup of the sugar together in a small measuring cup or bowl and set aside.

Mix flour, soda, and salt in another bowl and set aside as well.

In a large mixing bowl, beat butter and remaining 1 cup of sugar until smooth, about 2 minutes. Add eggs one at a time, beating after each addition. With mixer on low, add the flour mixture alternately with the milk, in 3 additions and then beat for 2 minutes. Stir in chocolate mixture and vanilla until completely incorporated.

Pour the batter into the prepared pan and bake for 18-22 minutes, rotating halfway through, until the cake springs back when touched in the center.

Cool in pan 10 minutes and then invert onto a cooling rack large enough to hold it; remove paper and let cool completely.

While the cake is baking, make the bark by melting the dark chocolate (low heat on stovetop or 1 to 1 1/2-minutes microwaved) stirring until smooth.

Melt the 4 ounces of white chocolate in a separate container.

Stir the puffed cereal into the dark chocolate and quickly spread on a parchment-lined baking sheet to about 1/4-inch thick (the edges don't have to reach the pan edges - just evenly spread until it's 1/4-inch). Immediately drizzle white chocolate over the entire top and then swirl into the dark chocolate with the tip of a butter knife to create a marbled effect. Place pan into the refrigerator to set.

While cake is cooling, prepare frosting for filling by melting the white chocolate and cream together (again, your choice of low heat on stovetop or 1 minute in the microwave) until smooth. Let cool about 15 minutes.

Beat butter and sugar together with a mixer just until incorporated and then add the cooled chocolate mixture. Beat on high speed until smooth and fluffy, about 2 minutes.

Cover with a moist towel or plastic wrap and set in the refrigerator until cake is cool. Let sit 10 minutes at room temperature if too cold, stirring until it's a spreading consistency.

Melt chocolate, butter, and honey together using the microwave or setting a bowl into a pan of barely simmering water to make a water bath.

Stir until smooth. It should be the consistency of heavy cream - if it's too thin, refrigerate for 5 minutes or so and stir to thicken slightly.

Cut the cooled cake into three even pieces, each about 5  1/2 x 11 inches. Set the first layer on a serving plate and spread with half of the white chocolate filling. Top with second layer (trim edges as needed to make even with bottom layer - I found it's really hard to get them even the first time cutting) and spread with remaining filling. Top with the last later and trim if needed.

Tear waxed paper into strips and tuck under all the edges of the cake. Stir ganache and then pour over the top of the cake, using a spatula to spread gently over the edges and onto the sides. Ganache will pool on waxed paper a bit - use spatula to lift up and spread onto sides as needed.

Let cool until ganache has just set - about 10 to 15 minutes - and gently remove waxed paper pieces.

Break the bark into small pieces and add to the length of the cake on top. Use some of the remaining bark around the edges of the cake.

Refrigerate until needed (can be made a day ahead).

Tip for cutting: remove the bark along cutting lines, cut slices, and then add bark back to the individual slices on each person's plate.