Preheat your oven to 350°F (175°C). Line an 8x8-inch (20x20 cm) baking pan with parchment paper or spray it with cooking spray.
In a bowl, whisk together the spelt flour, cocoa powder, baking powder, and salt.
In another bowl, combine the melted coconut oil (or butter), sugar, eggs, and vanilla extract. Mix until well incorporated.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
If you're using 100% whole grain spelt, you need to add the 1/8 cup of water. Let the batter sit for about 15 minutes as whole grains are thirsty and it will need to absorb the extra liquid. (If you're using store-bought spelt, you can skip this step.)
Mix in the chocolate chips and if using, fold in the walnuts or chocolate chips.
Pour the batter into the prepared baking pan, spreading it out evenly.
Bake for 20-25 minutes, or until a butter knife or toothpick inserted into the center comes out mostly clean.
Allow to cool before cutting into squares.