For the buns: add warm milk to the bowl of a stand mixer fitted with the dough hook attachment; sprinkle yeast over top. Whisk together 4 tablespoons melted butter, honey, egg, and egg yolk; add to bowl with milk and yeast and mix with a wooden spoon until combined. 

Whisk together flours, cornstarch and salt; add to bowl with wet ingredients and mix on low until a dough comes together, 1–2 minutes. Increase speed to medium-low and knead until a smooth, sticky dough comes together, about 8 minutes. 

(You can also use your hands by kneading the dough on a lightly-floured surface for 8–10 minutes.)

Transfer dough ball to an oiled bowl, cover bowl loosely with a kitchen towel and let rise for 1–1½ hours, or until doubled in size. 

Meanwhile, grease a 9-inch round or square cake pan, line with parchment paper, and grease the parchment paper. 

Transfer dough to a floured surface and roll out into a 14×9 inch rectangle with the long side parallel to the counter’s edge. 

Brush 1 tablespoon melted butter over dough, leaving ¼-inch border on farthest edge. 

For the filling, combine remaining 2 tablespoons melted butter with ¾ cup coconut sugar, cinnamon, vanilla, and salt. Evenly sprinkle filling mixture over dough, leaving a ¼-inch border on farthest side. 

Using a bench scraper, loosen dough from counter and roll away from you into a tight log; pinch seam to seal. Roll log seam side down and cut into 9 equal; pieces using a serrated knife (or floss). 

Reshape rolls as needed. Place one roll in the center of the prepared cake pan, then place remaining rolls around perimeter of center roll, seam side facing in (if using a round cake pan). For a square cake pan, arrange rolls in rows. Cover pan loosely with a kitchen towel and let rise until dough springs back when gently pressed, 30–45 minutes 

Heat oven to 350ºF with rack set in middle position.

Brush rolls with milk and bake until buns are golden brown on the edges, 23–25 minutes. Transfer pan to a wire rack and let cool 5 minutes. 

Meanwhile for the frosting, grind ¼ cup coconut sugar in a spice grinder to a powder. Beat together cream cheese, vanilla and salt, in stand mixer fitted with the whisk attachment. Add ground sugar and continue to beat until light and smooth.

Spread frosting over buns and serve.