Step 1: Prepare the Chocolate Whoopie Pie Cookies

1. Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.
2. In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside.
3. In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.
4. Mix in the egg and vanilla extract, beating until smooth.
5. Alternate adding the dry ingredients and buttermilk, mixing until just combined. Do not overmix.
6. Scoop tablespoon-sized mounds of batter onto the prepared baking sheet, spacing them about 2 inches apart.
7. Bake for 10-12 minutes, or until the cookies spring back when gently touched.
8. Let the cookies cool completely before assembling.

Step 2: Make the Coconut-Pecan Filling

1. In a medium saucepan over medium heat, whisk together evaporated milk, egg yolks, and brown sugar.
2. Add the butter and stir constantly until the mixture thickens (about 5-7 minutes).
3. Remove from heat and stir in the vanilla, shredded coconut, and chopped pecans.
4. Allow the filling to cool completely before using. It will thicken as it cools.

Step 3: Assemble the Whoopie Pies

1. Spread a generous spoonful of the coconut-pecan filling on the flat side of one cookie.
2. Press another cookie on top to create a sandwich. Repeat for all cookies.
3. Optional: Drizzle melted dark chocolate over the assembled whoopie pies for extra indulgence.
4. Serve immediately or store in an airtight container at room temperature for up to 2 days. Refrigerate for longer freshness.