Prep
Preheat oven to 350°F (175°C). Grease or line an 8×8-inch baking pan with parchment paper.
In a small bowl, mix brown sugar and cinnamon for the swirl. Set aside.
Make the Batter
In a large bowl, whisk flour, baking powder, baking soda, salt, and cinnamon.
In another bowl, mix plant-based milk, maple syrup, oil, vanilla extract, and apple cider vinegar.
Pour wet ingredients into dry ingredients and stir just until combined—do not overmix.
Assemble
Pour half of the batter into the prepared pan.
Sprinkle evenly with the brown sugar–cinnamon swirl mixture.
Add remaining batter and gently spread.
Crumb Topping
In a bowl, mix flour, brown sugar, cinnamon, and oil until crumbly.
Sprinkle evenly over the batter. Add nuts if using.
Bake
Bake for 35–40 minutes, or until a toothpick inserted in the center comes out mostly clean.