Prep: Preheat the oven to 350 F. Lightly grease and then line a 8” square pan or a 9” round cake tin with parchment paper, leaving a little paper hanging over two opposite sides to create flaps. Mix the soy milk and apple cider vinegar together in a small glass to create a buttermilk; set aside to let sit for at least 5 minutes.

Make the Crumble: Use a fork to mix the melted butter, flour, sugar, and cinnamon together in a small bowl until a crumbly mixture forms. Place the crumble in the fridge while you prepare the remaining ingredients.

Dry Ingredients: Whisk the flour, baking powder, baking soda, and salt together in a large bowl. Create a well in the center of the bowl.

Wet Ingredients: Add the melted oil or butter, prepared buttermilk, sugar, applesauce, and vanilla to the center of the bowl and slowly incorporate it into the dry ingredients until a mostly uniform dough forms (a few lumps are ok).

Assemble: Note: I like to divide my batter into two separate bowls for better results, but you can also just eyeball it. Transfer half of the batter to the prepared baking pan and use an offset spatula to spread it evenly across the base. Measure out 1 cup of crumble (it should be slightly less than half of the mixture) and use your hands to evenly sprinkle it over the batter. Use a spoon to dollop the remaining batter over the top of the crumble, then spread it out into an even layer with a spatula. Top the second layer of batter evenly with the remaining crumble.

Bake: Bake in the middle rack of the oven for 30 to 35 minutes, or until a toothpick comes out of the center of the cake cleanly. Set aside and let cool for at least 10 minutes before slicing.

Icing Drizzle (Optional): While the cake is cooling, whisk the powdered sugar, vanilla, and milk together in a small bowl until evenly combined. Drizzle over the coffee cake before slicing.

Serve & Store: Slice the coffee cake into 9 even squares in the pan, then carefully lift out of the pan using the parchment paper slaps. Serve warm, or let cool completely before serving. Store leftovers at room temperature for up to 3 days, in the fridge for up to a week, or in the freezer for up to 2 months.