Preheat the oven to 350°F and light-colored baking sheets with parchment paper.
Whisk dry ingredients: In a medium bowl, whisk together the flour, baking soda, corn starch, and salt.
2¼ cups all purpose flour, 1 teaspoon baking soda, ½ teaspoon salt, 1 Tablespoon cornstarch
Cream butter and sugar: In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter, brown sugar, and granulated sugar until light and creamy, 2-3 full minutes.
3/4 cup unsalted butter, 3/4 cup light brown sugar, 1/4 cup granulated sugar
Add egg and vanilla: Add the egg, egg yolk, and vanilla, and beat for another 1-2 minutes, until light and fluffy, scraping down the sides of the bowl as needed.
1 large egg + 1 egg yolk, 1 teaspoon vanilla extract
Finish the dough: With the mixer on low, slowly add the dry ingredients and mix until combined. Stir in the chopped chocolate and chopped nuts using a rubber spatula.
8 oz semi-sweet chocolate bar, ½ cup chopped roasted almonds
Stuff with marshmallow cream: Use a large cookie scoop (or a scant 1/4 cup) to scoop the dough. Break them in half and fill them with a heaping teaspoon of marshmallow cream. Running the measuring spoon under hot water between scoops prevents it from sticking too much. Close the cookie dough seams, leaving some of the marshmallow fluff showing on the tops of the cookies.
1/2 cup marshmallow cream
Bake: Arrange the cookies 2-3 inches apart on the prepared baking sheets. Bake for 10-12 minutes, until the edges are slightly golden and the tops are just set. Swirl a large-rimmed cup or round cookie cutter around the edges when they come out of the oven to gently nudge them back into a round shape if they spread at all during baking. Press extra chocolate chunks and almonds into the tops of the warm cookies. Cool for 5 minutes on the baking sheet then transfer to a wire rack to cool completely.
Toast the tops: Use a kitchen torch on the lowest setting to to toast the marshmallows for 1-2 seconds.