Grease a 9x13-inch pan and set aside.
In a large separate bowl whisk together the All-Purpose Flour (3 1/4 cup), Quick Cooking Oats (1 3/4 cup), Granulated Sugar (1 1/4 cup), Ground Cinnamon (2 teaspoon) and zest from the Sunkist Orange (1). Knead the Unsalted Butter (1 1/8 cup) into the dry ingredients until it forms streusel crumbs.
Press half the streusel mixture into the prepared pan.
Place the Semi-Sweet Chocolate Chips (1/2 cup) in a heatproof bowl. Heat Espresso (1/2 cup) until boiling and pour over the chocolate chips. Stir to smooth out the mixture.
Separate the Egg (4). Beat the egg whites and Salt (1/4 teaspoon) until stiff peaks form. Sprinkle the Granulated Sugar (3/4 cup) over the egg whites.
Add the egg yolks, Almond Butter (1/3 cup), Ground Nutmeg (1 teaspoon) to the espresso chocolate. Mix thoroughly. Fold in the sugared egg whites, then fold in the Buckwheat Flour (1 1/4 cup).
Preheat the oven to 350 degrees F (176 degrees C).
Spread the mixture out on top of the streusel in the pan. Distribute the rest of the streusel on top. Place in the oven and bake for 30 minutes.
Place the Espresso (8 tablespoon), Powdered Confectioners Sugar (1 cup) and Coffee Liqueur (2 tablespoon) into a saucepan and cook, stirring until dissolved. Bring to a boil, then remove from heat.
Remove cake from the oven and let cool for 10 minutes before drizzling the syrup over the top.