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Preheat oven to 350F. Line 2 baking sheets with parchment paper.
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Place semisweet chocolate chips in a small microwave-safe bowl. Microwave in 30 seconds increments, stirring after each one, until melted. Set aside to cool completely.
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In the bowl of a stand mixer fitted with a paddle attachment, beat butter and sugar together until lightly and fluffy. Beat in eggs and vanilla.
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In a separate bowl, whisk together flour, baking powder and salt. Add this to the butter mixture and beat until just combined. Use a rubber spatula to cook in pecans and milk chocolate chips.
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Drop by tablespoonfuls onto the prepared baking sheets and top with a few marshmallows. Bake in preheatened oven for 10 - 12 minutes. Cool on baking sheet for a couple minutes before transferring to a wire rack to cool completely.