In a medium bowl whisk together starter, water, honey, and olive oil until starter and honey have dissolved and mixture is bubbly.
In a large bowl or the bowl of a stand mixer whisk together flours and salt. Add wet ingredient mixture to dry ingredients and mix to form a shaggy dough.
Knead dough until soft, supple, and slightly tacky (but not sticky). This may take about 6-8 minutes by hand or 5-6 minutes in a mixer with dough hook on medium-low speed.
Transfer dough to a clear container or bowl. Mark the doughs position on the side the container with a washable maker or rubber band and cover.
Leave dough to rise at room temperature until it has increased in volume by about 50-100%. This may take around 4-6 hours (see note).
Line a half sheet pan (13x18 inches) with parchment. Fill a small bowl with water and use it to wet your hands. Punch down dough then gently release from the sides of the container and transfer to a lightly floured work surface. Divide dough into 8 equal portions with a knife or bench scraper (for best results, weigh portions for accuracy).
Pinch and pull the edges of each portion of dough into the center until taut balls form.
Arrange buns evenly spaced on sheet pan. Cover and allow to rise until about doubled in volume, this may take around 3-6 hours (see note).
When ready to bake preheat to 350°. In a small bowl whisk together egg and heavy cream. Brush top of buns with egg wash. Top generously with sesame seeds if desired. Bake 25-35 minutes until golden brown and internal temperature is at least 200°F.
Leave on the pan for 10 minutes then remove to a wire rack to cool completely before serving.