Experiment No. 202 — Sweet Potato Pie Crust Cookie

Scope:
Recreate the flavor architecture of Southern sweet potato pie inside a cookie format—crisp edges, pie-style bottom crust texture, soft custard-like center, and a toasted marshmallow plug that melts into the filling.
Objective:
Engineer a cookie that behaves like a mini slice of pie: a stabilized sweet-potato custard filling, a butter-forward crust-cookie base, and a structural marshmallow core that prevents leakage and enhances chew.

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Time & Yield:
Prep: 25–30 min
Chill: 30 min
Bake: 12–15 min
Rest: 10 min
Yield: 12 cookies (55–60 g each)

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Formula — Weight, Volume, Ratio %

Weight = accuracy
Volume = accessibility
Ratio % = proportion of total dough/filling mass

Cookie Base (Crust Behavior)
Unsalted butter: 113 g (½ cup), 20%
Brown sugar: 100 g (½ cup), 18%
Granulated sugar: 75 g (⅓ cup), 13%
Egg: 1 large (50 g), 9%
Vanilla extract: 8 g (2 tsp), 1%
All-purpose flour: 210 g (1¾ cups), 37%
Baking powder: 4 g (1 tsp),
Fine salt: 4 g (¾ tsp)

Sweet Potato Filling
Cooked mashed sweet potato: 150 g (⅔ cup), 55%
Brown sugar: 40 g (3 Tbsp), 15%
Butter (melted): 14 g (1 Tbsp), 5%
Egg yolk: 18 g (1 yolk), 6%
Cinnamon: 2 g (½ tsp),
Pumpkin spice: 1 g (¼ tsp),
Salt: pinch
Vanilla: 4 g (1 tsp)
Ratio note: Higher mash % = pie-like density; yolk binds and adds custard chew.

Marshmallow Plug
Large marshmallows: 12 units
Cut a small slit in each to allow controlled melt.

Procedure

1. Prepare the sweet potato filling.
   Whisk mashed sweet potato, brown sugar, melted butter, egg yolk, cinnamon, pumpkin spice, vanilla, and a pinch of salt. Mix until smooth and glossy. Let cool—it will thicken as starches set.

2. Mix the cookie base.
   Cream butter + both sugars until fluffy. Add egg + vanilla.
   Fold in flour, baking powder, and salt until a soft dough forms.
   Chill 20 minutes for structure.

3. Assemble the cookies.
   Scoop 55–60 g dough balls.
   Press a deep well into the center with your thumb or a spoon.
   Place one marshmallow plug inside the well.
   Spoon about 1 tablespoon of sweet-potato filling over the marshmallow and smooth the top.

4. Bake.
   Bake at 350°F for 12–15 minutes until edges are golden and centers are just set.
   Cool 10 minutes before moving—this allows the marshmallow to contract and the filling to firm.

5. Finish.
   Optional flaky salt on top or a light cinnamon-sugar dust while warm.

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Science Notes

Sweet potato puree contains natural starches that thicken as they cool—this is why the filling must be cooled before assembly.
The marshmallow plug prevents the filling from absorbing into the dough, acting as a moisture barrier and creating a molten-goo center once baked.
Chilling dough limits spread and maintains crust-like edges.
Egg yolk in the filling adds lecithin, which emulsifies fat and creates custard behavior inside a cookie.

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Variants

Classic Pie Spice: Increase pumpkin spice to 1 teaspoon for a deeper holiday flavor.
Brown Butter Base: Brown the butter first for extra nuttiness; chill until solid before mixing.
Maple Glaze: Whisk ¼ cup powdered sugar + 1 tablespoon maple syrup and drizzle on cooled cookies.
Vegan Option: Swap butter for vegan butter, egg for flax egg, and marshmallow for vegan marshmallow.
Cheesecake Core: Replace marshmallow with 1 teaspoon cream-cheese filling for a swirled effect.