In a medium bowl, sift together 1 cup unsweetened cocoa powder, 2 cups all-purpose flour, 1 teaspoon baking powder, and 1/2 teaspoon salt. Set aside.
In a large bowl, whisk together 1 cup granulated sugar, 1/2 cup packed brown sugar, and 1/2 cup vegetable oil until smooth and well combined.
Beat in 4 large eggs, one at a time, then stir in 1 teaspoon vanilla extract.
Gradually fold the dry ingredient mixture into the wet ingredients using a spatula or wooden spoon. Mix just until combined.
Cover the dough and refrigerate for at least 2 hours, or overnight if possible.
Preheat oven to 350°F (175°C). Scoop tablespoon-sized portions of dough and roll into balls. Coat each ball generously in 1 cup powdered sugar.
Place dough balls on a parchment-lined baking sheet about 2 inches apart. Bake for 10-12 minutes until edges are set but centers remain soft.
Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Spread about a tablespoon of chocolate frosting on the flat side of one cookie, then gently press another cookie on top to form a sandwich. Repeat.
Serve immediately or store as desired.