Prepare the whoopie pie cakes:

• Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats.
• In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
• In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
• Beat in the egg and vanilla extract until combined.
• Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.
• Stir in the boiling water to thin the batter to a pourable consistency.
• Drop spoonfuls of batter (about 1-2 tablespoons each) onto the prepared baking sheets, spacing them about 2 inches apart.
• Bake for 10-12 minutes, or until a toothpick inserted into the center of a cake comes out clean.
• Allow the cakes to cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely.

Make the coconut-pecan filling:

• In a medium saucepan, combine the butter, granulated sugar, evaporated milk, and lightly beaten egg. Cook over medium heat, whisking constantly, until the mixture thickens, about 5-7 minutes.
• Remove the saucepan from the heat and stir in the vanilla extract, shredded coconut, and chopped pecans. Let the filling cool for 10-15 minutes, or until it reaches a spreadable consistency.

Assemble the whoopie pies:

• Once the whoopie pie cakes have cooled completely, spread a generous amount of the coconut-pecan filling on the flat side of one cake.
• Top with another cake, flat side down, to form a sandwich.
• Repeat with the remaining cakes and filling.

Serve:

• Serve immediately or refrigerate the whoopie pies for an hour to allow the filling to firm up before serving.