1 1/4 cups unbleached all-purpose flour
3/4 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
2 1/4 cups old-fashioned oats
1 cup dried tart cherries, chopped
1 cup pecans, chopped
3/4 cup semisweet or bittersweet chocolate (4 ounces), chopped into chocolate chip–size chunks
3/4 cup (1 1/2 sticks) unsalted butter, room temperature
1 1/2 cups (packed) dark brown sugar
1 large egg
1 teaspoon vanilla extract
2 7-ounce jars marshmallow creme
Chocolate dipping sauce:
1/2 cup semisweet or bittersweet chocolate (2 1/2 ounces), chopped
1 tablespoon honey
1/2 cup heavy cream
Arrange racks in upper and lower thirds of oven; preheat to 350°F. Line 3 baking sheets with parchment paper. Whisk flour, baking powder, baking soda, and salt in a medium bowl. Combine oats, cherries, pecans, and chocolate in a large bowl.
Using an electric mixer, beat butter and sugar in another large bowl, occasionally scraping down sides of bowl, until smooth, about 2 minutes. Add egg and vanilla and beat to blend. Scrape down sides of bowl. With machine running at low speed, gradually add flour mixture; beat just to blend. Stir in oat mixture with a wooden spoon just to blend.
Using a 1-ounce ice cream scoop or scooping out 2 level tablespoonfuls of dough, measure dough into 36 portions. Divide among baking sheets. Using your hands, gently press down on each dough ball until 1/2″ thick.
Bake cookies for 12 minutes. Rotate sheets front to back and top to bottom; continue baking until cookies are golden brown and edges have begun to set but centers are still soft (cookies will appear underdone and shiny in centers), 7–8 minutes longer. Do not overbake. Let cookies cool on baking sheets. DO AHEAD: Cookies can be made 2 days ahead. Store airtight at room temperature.
Spread 1 tablespoon marshmallow creme on bottom of 18 cookies. Sandwich with remaining cookies.
For chocolate dipping sauce:
Place chocolate and honey in a medium bowl. Bring cream just to a boil in a small saucepan. Pour over chocolate and honey; let stand for 1 minute; stir until melted and smooth. Drizzle over cookies or serve alongside sandwich cookies for dipping.