In a bowl, whisk flour, cocoa powder, baking powder, baking soda, and salt. Set aside.

In a bowl of a stand mixer, add butter and sugars, and beat until creamy, 3-4 minutes. Add egg and egg yolk one at a time, beating well after each addition. Add in vanilla and oil and mix for 1 minute.

Add dry ingredients, beat until just incorporated until no flour streaks remain. Don't overmix. Wrap in plastic foil and refrigerate for 5 hours, or overnight.

Preheat oven to 180°C (350°F) and line two baking sheets with baking paper.

Place the sugar in a small bowl.

Roll about 10 grams of dough into little balls and then roll them in sugar. Place od the prepared baking sheet (or plate lined with baking paper if the baking sheet is too large to fit in the freezer) and put in the freezer for about 15 minutes.

Bake cookies for 7-8 minutes. Let cool for 5 minutes and transfer to a rack to cool completely.

For caramel, heat granulated sugar in a medium saucepan over medium heat, stirring constantly with a wooden spoon.
The sugar will clump up first, but will eventually completely melt. Once sugar is completely melted, immediately add the butter and whisk until combined. Be careful because the mixture will bubble up. Stir the butter into the caramel until it is completely melted. Slowly pour the heavy cream into the caramel and whisk until incorporated. Allow the mixture to boil for 1-2 minutes. Leave to cool and harden a bit.

Spread about 1 teaspoon of caramel on one half of the cookie and press the other cookie on top. Repeat with the remaining cookies.

Store in the fridge in an airtight container for up to 5 days, but take them out 20-30 minutes before serving.