Preheat oven to 350℉ and line an 8x8 square pan with parchment paper.
First prep the streusel. In a medium bowl, mix butter, flour, brown sugar and salt until just combined. Set aside.
Next, make the espresso swirl. In a small bowl, mix together the sugar, espresso powder and cinnamon. Set aside.
Then, make the cake. In a large bowl, whisk oil and sugars together.
Add egg and vanilla. Whisk until smooth.
Add sour cream. Whisk until smooth.
Add flour, cornstarch, baking powder, salt and espresso and fold with a spatula until just combined.
Pour half of the cake batter into the prepared tin, spreading into an even layer.
Sprinkle over the espresso swirl then dot small spoonfuls of the remaining batter all over the top, covering as much of the swirl mixture as possible before gently spreading so that all of the swirl mixture is covered.
Sprinkle the streusel on top in an even layer.
Bake for 45-50 minutes or until a skewer inserted into the middle of the cake comes out clean.