Preheat oven to 375 degrees Fahrenheit. Line a 10×15-inch sheet pan with parchment paper or grease it well.

Add the lemon juice to a measuring cup or small bowl. Pour the soy milk over it and set aside.

In a large saucepan, combine the peanut butter, water, and margarine. Bring to a boil over high heat, stirring constantly.

Remove from the heat and add the granulated sugar, light brown sugar, flour, salt, and baking soda. Mix until well combined.

Stir the soy milk/lemon juice. It should be curdled but still runny. This is your pareve buttermilk.

Add the “buttermilk,” eggs, and vanilla extract to the saucepan. Mix until combined.

Pour the batter into the prepared sheet pan and bake for 20 minutes, until a toothpick inserted in the center comes out clean.