In the bowl of a stand mixer, whisk together the buttermilk, yeast, and 1 teaspoon of the granulated sugar. Let sit for 5 minutes.
Add the remaining 2 tablespoons of granulated sugar, egg, softened butter, salt, and 1 cup of the flour into the bowl. Mix on low speed with the paddle attachment until everything is well combined.
Use a spatula to scrape down the sides of the bowl. Gradually add the remaining flour until the dough comes together. At this point, switch to the dough hook and mix for about 2 minutes.
If the dough feels sticky, add a little more flour, one tablespoon at a time, until you achieve a slightly sticky but workable dough.
Transfer the dough onto a lightly floured surface. Knead by hand for about 3 to 5 minutes, or use the dough hook to knead in the mixer.
Spray a large bowl with cooking spray and place the kneaded dough inside. Cover with plastic wrap and let rise for about 1 hour or until doubled in size.
Once risen, punch the dough down and divide it into 14 to 16 equal pieces.
Take each portion of dough and stretch it over itself, pinching at the bottom to create a smooth top. Roll each piece until round.
Place them in the prepared pan, leaving space between each.
Cover again and let rise for about 1 hour until doubled in size.
Preheat the oven to 350 degrees F. Bake for 20 to 25 minutes or until golden brown. Brush with butter and sprinkle with flaky sea salt.