• Preheat the oven to 350 F (180 C) for conventional oven. Lower the temperature by 20 F (15 C) if using a convection oven.
• You can bake this recipe using 9-inch springform pan or 8 x 8 inch square pan. If using a springform pan, line the bottom round with parchment paper. If using 8-inch square pan, line with parchment paper sling, overhanging on all sides
• You can also use 9-inch square pan. The baking time will be slightly shorter. Start checking at 50 minutes
• If you want to do 1/2 of the recipe, bake in a 6-inch springform pan or a pan with a loose bottom.

Prepare the cocoa streusel:

• Melt the butter and let it cool until it is just warm. Combine brown sugar, flour, salt, and cocoa powder in a mixing bowl and whisk to mix thoroughly. Add melted butter and use a fork to stir to let it mix into small and large clumps. Cover and put this in the freezer while you prepare the cake batter
  

Prepare the cake batter:

• Combine the flour, baking powder, baking soda, and salt in a mixing bowl and set aside
• In another mixing bowl, combine milk, oil, toasted sesame oil, eggs, and ground black sesame. Whisk until well-combined
  
• Add the flour mixture and use a spatula to gently fold until combined.
  
• The batter may be lumpy, but as long as you don't see anymore pockets of flour, you are done mixing. You don't want to overmix
  
• The batter is thick but flowy. Pour the batter into the prepared pan. Tap the pan on the counter a few times to knock out any large air bubbles trapped inside the cake batter
  
• Remove the streusel from the freezer and use your hands to gently break up some large clumps. Sprinkle this evenly over the batter. Gently use your fingers to lightly press on the streusel. This helps the streusel to stick to the cake later after baking
  

Baking:

• This recipe yields a pretty tall cake, so the baking time does take a bit longer because of the height.
• Bake in the middle rack for 65-70 minutes or until a skewer inserted into the center of the cake comes out clean with just a few sticky crumbs. It shouldn't be wet. If it is, bake another 5 minutes and check again. Every oven can be so different, so start checking at around 60-minute mark

Cooling:

• Place the pan on a cooling rack and let the cake cool on the rack for 10 minutes and then loosen the sides of the cake using an offset spatula if you are using a springform pan. If you are using a square pan, simply grab on the parchment paper to lift the cake out of the pan onto a cooling rack. Let it cool down completely before cutting and serving
  
• This cake tastes best the next day to be honest! The flavor has time to develop and the cake is moister too. Once the cake has cooled down completely, I store the whole uncut cake back into the pan without a cover (to preserve the crunchy top). Then serve it the next day!