Grind the 60 g of oats in a food processor or blender until they look like a coarse flour. Whisk together with the all purpose flour, whole oats, baking soda, baking powder, salt, cinnamon, and nutmeg. Set aside.

Cream the butter with both sugars and the molasses on medium with the paddle for a full 4 minutes, scraping down twice, until pale and light. Do not shorten this step.

Add the eggs one at a time, then the yolk, beating well after each. Add the vanilla and the sourdough discard and mix until smooth and glossy.

Add the dry mixture all at once and mix on low just until no dry flour remains. The dough will be thick and sticky. This is correct. Do not add extra flour.

Cover the dough and refrigerate at least 2 hours, or up to 48 hours for a longer ferment. This chill is required, not optional. The dough is too sticky to handle straight from the mixer.

Line two quarter sheet pans with a parchment sling that hangs over the two long sides. Do not grease underneath.

Weigh 780 g of chilled dough into each pan, dropping it in six or eight separate blobs spread around rather than one mound in the center.

Lay a sheet of plastic wrap loosely over the dough and roll it level right in the pan with a small rolling pin, working the corners deliberately. Peel the plastic away.

Heat the oven to 350°F (175°C).

Bake 18 to 22 minutes, checking at 18. Pull when the edges are set and just releasing from the parchment and the center still looks slightly underdone. It will not look finished. Take it out anyway.

Rap each pan firmly on the counter once or twice the moment it leaves the oven to settle any doming.

Cool 10 minutes in the pan, then lift both slabs out by the parchment onto racks and cool completely.

Beat the filling butter and shortening together until smooth and light, about 2 minutes. Add the marshmallow creme and beat 1 minute more. Add the powdered sugar, vanilla, and salt, then whip on medium high 2 minutes until fluffy.

Chill both cooled slabs in the refrigerator 30 minutes. Invert the first slab onto a board so the craggy baked side faces down and the smooth pan side faces up, then peel off the parchment.

Scoop the filling across the slab in dollops and spread it into an even layer right out to the edges.

Lay parchment over the second slab, set a flat board on top, and flip it so the slab is craggy side down. Peel off the parchment underneath, flip once more onto a second board so it is craggy side up, then line up one long edge and slide it onto the filling while drawing the board back out.

Press gently with a flat sheet pan to seat and level everything.

Cover with plastic wrap and freeze 1 hour.

Cut while still frozen. Trim ¼ inch from all four sides, then cut 4 columns down the length and 6 rows across the width. Press straight down rather than sawing and wipe the blade between cuts.

Let the bars sit at room temperature 20 to 30 minutes before serving.