Grease 9x5-inch baking pan and preheat oven to 350°F if baking immediately.
In a large bowl mix dry ingredients: flour, baking soda, baking powder, salt.
In another bowl, whisk or mix together brown sugar and butter until fluffy.
Add eggs one at a time to mix, then maple syrup, sourdough starter, lemon juice and zest, vanilla, and sour cream.
Add the wet to the dry and fold in until just incorporated.
If you are wanting to ferment batter, cover with plastic wrap and refrigerate for several hours or overnight.
Pour into prepared pan and bake for approximately 60 minutes, testing with a toothpick at 50 minutes to see if comes out clean.
Set aside to cool for an hour or longer.