Preheat and Prepare:

Preheat oven to 350°F (175°C). Line two large baking sheets with parchment paper.

Mix Dry Ingredients:

In a medium bowl, whisk together flour, baking powder, cornstarch, and salt. Set aside.

Cream Butter and Sugars:

In a large bowl, beat butter, granulated sugar, and brown sugar until creamy and fluffy (about 2 minutes).

Add Wet Ingredients:

Beat in the egg, egg yolk, and vanilla until smooth.

Combine the Dough:

Gradually add dry ingredients to wet, mixing until combined. The dough should be soft but not sticky.

Form Cookies:

Scoop about 2 tablespoons of dough per cookie and roll into balls. Place 2 inches apart on baking sheets.

Create Thumbprints:

Use your thumb or the back of a measuring spoon to press a deep indentation in the center of each ball. Chill for 10 minutes if dough feels soft.

Bake the Cookies:

Bake for 8–10 minutes, until edges are set but centers are still pale.

Prepare the Filling:

In a saucepan, combine corn syrup, brown sugar, butter, cinnamon, salt, cloves, and nutmeg. Bring to a gentle boil over medium heat, stirring constantly. Boil for 1 minute, then remove from heat.

Add Flavor and Nuts:

Stir in vanilla extract, heavy cream, and chopped pecans. The mixture will thicken as it cools slightly.

Fill and Finish:

Spoon about 1 teaspoon of filling into each cookie’s indentation. Return cookies to oven for 5 minutes to set the filling.

Cool and Serve:

Allow cookies to cool on the pan for 10 minutes before transferring to a rack. Enjoy warm or at room temperature.