In a large bowl or stand mixer, cream together softened butter, powdered sugar, and brown sugar until light and fluffy.

Add the egg, vanilla extract, and almond extract if using. Mix until well combined.

Add the sourdough starter and mix until fully incorporated.

In a separate bowl, whisk together flour, baking powder, salt, and cream of tartar.

Gradually add the dry ingredients to the wet ingredients and mix until just combined. Do not overmix.

Wrap the dough tightly in plastic wrap and refrigerate.

For the crumble, add flour, brown sugar, powdered sugar, and salt to a food processor and pulse a few times to combine.

Add the graham cracker pieces and pulse until you have a mix of larger chunks and fine crumbs with real texture.

Add the cold cubed butter and pulse until incorporated.

Add the sourdough starter and vanilla extract and pulse until the mixture reaches a brown sugar texture. It should hold together when squeezed and crumble apart easily when broken.

Transfer the crumble to a container, cover, and refrigerate alongside the dough for a minimum of 2 hours and up to 48 hours.

Toss the mini marshmallows with the cornstarch until lightly coated. Place in the freezer for at least 30 minutes before assembly.

Preheat oven to 350 degrees F (175 degrees C).

Line a 13x9 inch baking pan with parchment paper, leaving overhang on all four sides.

Optional, Break a few graham cracker squares from the crumble mixture into irregular pieces and scatter them loosely across the parchment. Leave gaps between the pieces.

Scatter the fermented crumble over and around the graham cracker pieces, spreading it across the entire pan. Push the crumble firmly all the way to the edges and corners of the pan to seal the border.

Scatter the chopped chocolate chunks evenly over the crumble layer.

Add the frozen cornstarch dusted marshmallows in an even layer over the chocolate.

Push the chocolate and marshmallows around so they break through to the bottom of the pan to have that beautiful marbled top.

Trace the bottom of your 13x9 pan onto a separate sheet of parchment paper. Use that outline as a guide and roll the cold fermented cookie dough to match the shape, approximately ⅜ inch thick.

Carefully transfer the dough slab into the pan over the s'mores layers. Press it down gently to cover everything completely, pressing into the corners and edges with lightly floured fingers.

Bake for 20 to 30 minutes until the edges are golden and the top of the cookie slab is fully set. The marshmallows will have puffed up the dough.

Remove from oven and let cool in the pan for a full 45 minutes before flipping.

Use a piece of parchment and lay it over top of the dough top. Place a large cutting board over the top and flip in one confident motion. Peel the parchment away carefully.

Let the bars continue cooling at room temperature until the chocolate is fully set before slicing.

Use a sharp knife and make clean decisive cuts. Wipe the blade between cuts for the cleanest edges.