Preheat your oven to 350°F and line an 8×8 inch pan with parchment paper.
In a bowl, melt the butter completely for the crumble.
Add sugar and cinnamon and whisk to combine.
Fold in flour until crumbly, then set aside.
In a separate bowl, whisk together flour, cocoa powder, baking powder, and salt.
In a large bowl, beat the butter, oil, and sugar until fluffy.
Add eggs one at a time, mixing well between each, then mix in the vanilla.
Gradually add the dry ingredients while slowly pouring in a mixture of milk and coffee.
Stir until smooth, then pour into the prepared pan.
Sprinkle the crumble evenly over the batter and bake for 30–35 minutes, until a toothpick inserted in the center comes out clean.
Let the cake cool completely in the pan or for 30 minutes if you prefer it slightly warm.
Finish with a drizzle of melted chocolate or a dusting of powdered sugar.