Pre-heat oven to 350F / 180C.
Mix the milk at room temperature with the vinegar and set aside (Side-note: if a recipe calls for buttermilk and you don't have any in hand, this milk + vinegar mixture acts as a great substitute for buttermilk)
In a bowl, sift together the flour, soda, cocoa powder, coffee powder, and salt. Mix the powdered sugar with this and stir well to combine
Mix the oil and vanilla to the milk + vinegar mixture that's been set aside
Now add the wet ingredients to the dry ingredients and gently fold to combine until there are no more streaks of flour left
Transfer the batter to a greased 8x8 pan and add chopped walnuts on top
Bake for 25-30 mins until a skewer inserted into the brownie comes out with crumbs
Let the baked brownies sit in the pan and cool completely before you cut into squares
Keeps well for up to 2 days outside or 4 days refrigerated