Line a loaf pan (I use an 8.5 x 4.5 Pyrex) with parchment paper and preheat the oven to 350 degrees. Have your baking rack positioned in the lower third of your oven.
Melt the butter over low heat and allow to cool slightly.
While the butter is melting, separate the eggs. Add the yolks to a large bow and the whites to a medium bowl.
Add the 1/4 teaspoon of salt to the egg whites and beat until soft peaks form. Leave aside until the end of the recipe.
Add the cooled butter and sugar to the egg yolks. Mix until lighter in color. If you are using a hand mixer, this will take about 3 or 4 minutes. It should take just 1 or 2 minutes if you are using a stand mixer.
Next, add the buckwheat flour, the pumpkin pie spice and the pumpkin puree. Mix until smooth.
Once it is smooth, add your sourdough starter and mix until just incorporated.
Finally, fold in the egg whites and any add ins if you have chosen some.
Pour the batter into your lined loaf pan and put it into the oven.
Bake for 55-60 minutes. It is ready when a toothpick or skewer comes out clean after being inserted into the loaf.
When the Buckwheat Sourdough Pumpkin Loaf comes out of the oven, allow it to cool in the pan on a baking rack for about two hours before cutting into it. Torture, I know, but worth it!!