Chocolate cake:

1. Pre-heat your oven to 350F and line a 13x18" half sheet pan with parchment paper on the bottom
2. Add the flour, granulated sugar, brown sugar, baking soda and salt to a large bowl. Whisk to combine and remove any lumps
3. Separately, add the butter, oil, cocoa powder and water to a saucepan. Place it on the stove on medium low heat. Cook and stir until the butter melts and a smooth sauce forms. Bring it to a simmer, and cook for another 1 minute. Then remove from the heat
4. Pour the hot cocoa mixture on top of the dry ingredients. Use a hand whisk to mix until combined. This will be a thick, slightly grainy batter - don't overmix
5. Add the eggs, buttermilk and vanilla. Whisk slowly and gently until a smooth chocolate cake batter forms. Do not overmix
6. Pour the batter into the prepared sheet pan. Bake for just 10-12 minutes or until a toothpick inserted in the middle of the cake comes out with light, moist crumbs (no wet batter). Avoid overbaking
7. Remove the cake from the oven and let it cool down

Chocolate frosting:

1. Toast the pecans in the pre-heated 350F oven for ~8 minutes or until crunchy and fragrant
2. Add the butter, cocoa powder, milk, salt and powdered sugar to a saucepan. Whisk and cook on medium low heat until the butter melts and a smooth sauce forms. Remove from heat, and stir in the pecans
3. Pour the frosting on top of the cake in the sheet pan and spread it out
4. Let the cake rest for 1-2 hours at room temperature until it forms a shiny film on top. Optionally, sprinkle over flakey sea salt. Slice, serve and enjoy!