In a mixing bowl, combine your streusel ingredients until they form a crumbly texture. Divide this mixture in half, adding cocoa powder to one half to create a rich swirl. Set both portions aside.

In a small bowl, dissolve instant coffee in 2 tablespoons of boiling water. Stir until fully mixed, then let it cool down to room temperature.

In a large bowl, whisk together the all-purpose flour, baking powder, salt, ground cinnamon, and cocoa powder (if using). Set this mixture aside.

In another bowl, whisk together the eggs, sour cream, vegetable oil, and your cooled coffee mixture until smooth.

Gradually incorporate the dry mixture into the wet ingredients using a spatula, mixing until just combined. Be careful not to overmix.

Preheat your oven to 350°F (175°C) and grease an 8x8-inch baking pan. Spread half of the cake batter evenly in the pan, followed by a sprinkle of the cocoa-streusel mixture. Top with remaining batter and finish with plain streusel.

Place the pan in the preheated oven and bake for approximately 40-45 minutes or until the top is golden brown and a toothpick inserted into the center comes out clean.

Once baked, remove the cake from the oven and let it cool completely. Prepare a glaze by mixing powdered sugar with brewed coffee, then drizzle over the cake.