In a medium mixing bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, baking powder, baking soda, and salt until well combined. In a stand mixer, beat the room temperature butter and both sugars for 3-4 minutes until creamy and light. Add the egg, egg yolk, vanilla extract, and oil, mixing until incorporated. Gradually add the dry ingredients until just combined, then wrap the dough in plastic wrap and refrigerate for at least 5 hours, or overnight, for the best texture.

When ready to bake, preheat your oven to 180°C (350°F) and line your baking sheets with parchment paper. Roll the chilled dough into small balls, about 10 grams each, and then roll each ball in turbinado sugar until well coated. Place the coated dough balls on the prepared sheets, and freeze for 15 minutes to firm them up before baking.

Place the baking sheets in the preheated oven and bake the cookies for 7-8 minutes. They should be slightly soft to the touch when you take them out, as they'll continue to set while cooling. Allow the cookies to cool on baking racks.

Prepare the caramel filling by adding the granulated sugar to a saucepan over medium heat, stirring constantly until melted into a golden caramel. Remove from heat, whisk in the unsalted butter until blended. Gradually stir in the heavy cream and cook for another 1-2 minutes, then add the salt.

To assemble, take one cooled cookie, spread about 1 teaspoon of the caramel filling on the flat side, and place another cookie on top. Repeat until all cookies are filled.