Preheat the oven to 350 degrees F.
Make the buttermilk first. To do this, mix the buttermilk ingredients together and set aside for 15 minutes. You will only need 6 tablespoons from this.
In a large bowl, cream together the sugar and the butter until fluffy.
Add in the buttermilk, half of it at a time and beat between each addition.
In a separate bowl, mix together the remaining dry ingredients.
Add in the flour mixture alternately with the ½ cup oat milk into the butter and sugar mixture until just combined, then stir in vanilla.
Lightly grease the cake pan with some neutral oil or vegan butter, then pour the cake batter in.
Bake for 40-45 minutes until a toothpick inserted in the center comes out clean.
Remove the cake from the oven and allow it to cool on a heat-proof surface.