Mix the flour, brown sugar, espresso powder, cinnamon and salt in a bowl

Add the cold cubed butter, and use your hands to rub it in until the mixture resembles crumbs. It's ok to have a few larger crumbs. You can also use a food processor for this step

Place in the fridge while you prepare the cake

Preheat oven to 350F (conventional) and line a 9x9" baking pan with parchment paper

Mix the sugar, melted butter, oil and vanilla in a large bowl for 2 minutes using a hand whisk

Add the eggs and egg white and whisk for another minute

Sift in the cake flour, salt, baking powder, and baking soda, and use a rubber spatula to gently fold until combined

Lastly, add the sour cream and hot brewed coffee, and fold until a smooth batter forms. Do not overmix

Pour half of the batter into the prepared pan. Then sprinkle over half of the coffee streusel. Gently spoon over the remaining half of the batter, spreading it out with a spoon or spatula, and top off with the remaining streusel

Bake for 30-35 minutes until a toothpick inserted comes out with moist crumbs attached

Let the cake cool in the pan for 15 minutes before taking it out1

Mix the powdered sugar, vanilla and coffee in a bowl until they form a smooth icing

Once the cake has cooled, drizzle over the icing, cut into squares, and serve!