Line a cookie sheet with parchment paper or silicone baking mat. Preheat oven to 350 degrees.
In a microwave-safe bowl, melt semisweet chocolate chips at 50% power in the microwave.
Using an electric mixer, beat butter and sugar at medium speed; add eggs one at a time, beating until blended after each addition. Add vanilla and melted chocolate and blend.
In a small mixing bowl, whisk flour, baking powder, and salt. Gradually mix flour mixture into chocolate mixture until well combined. Stir in pecans and 3/4 cup milk chocolate chips.
Drop dough by heaping tablespoonfuls onto the prepared cookie sheet and flatten them slightly. Once you have placed 12 cookies on the sheet, bake about 8 minutes in the preheated oven. Remove cookie sheet from the oven and place 4-5 marshmallows on the top of each cookie. Return the cookies to the oven and bake another 2-3 minutes or until marshmallows are slightly brown and fluffy.
Allow the cookies to cool on baking sheet for 3-5 minutes then transfer them to a wire rack to cool completely.
Melt the remaining 3/4 cup milk chocolate chips in a small microwave-safe bowl at 50% power in the microwave. Pour melted chocolate into a disposable piping bag and snip off the tip. Drizzle chocolate over the cookies. Allow the chocolate to set and harden completely. Serve.