In a large bowl, cream together the butter, granulated sugar, and brown sugar with an electric hand mixer.
3. Step
Add the eggs, one at a time, and beat until incorporated. Add the vanilla and beat until combined.
4. Step
Sift in the flour, cocoa powder, salt, and baking powder. Beat until combined.
5. Step
Divide the dough into 4 equal portions. Place a portion of dough between 2 sheets of parchment paper and roll out into a circle that is about 11 inches (27 cm) wide. Using a 10-inch (25 cm) springform pan as your guide, cut a 10-inch (25 cm) circle out of the dough.
6. Step
Use a 3-inch (8-cm) ramekin as a guide to cut out the center, leaving a doughnut-shaped cookie. Reserve the dough scraps and transfer the cookie on the parchment paper to a baking sheet. Repeat with the remaining dough portions.
7. Step
Chill the cookie dough in the freezer for 10 minutes.
8. Step
Bake the cookies (in batches, if necessary) for 12-14 minutes, until crisp around the edges. Let cool completely.
9. Step
Reroll the dough scraps and use a cookie cutter to cut out 24 1½-inch-wide (4-cm) cookies. Transfer the cookies to a baking sheet lined with parchment paper and chill in the freezer for 10 minutes.
10. Step
Bake the cookies for 7 minutes, until crisp. Let cool completely.
11. Step
Make the whipped cream: Add the heavy cream to a large bowl. With an electric hand mixer, whip the cream until soft peaks form, then add the sweetened condensed milk. Continue whipping until stiff peaks form. Transfer to a piping bag or zip-top bag fitted with a large round tip.
12. Step
Carefully place one of the large cookies on a serving platter. Pipe a layer of whipped cream on top of the cookie. Repeat with the remaining cookies and whipped cream, finishing with a layer of cream. Reserve any leftover whipped cream for sandwich cookie garnish.
13. Step
Chill the cake in the refrigerator for at least 4 hours or overnight.
14. Step
After the cake has chilled, make sandwich cookies for decoration. Pipe a dollop of whipped cream onto a small cookie and sandwich with another cookie. Make 9 more cookie sandwiches. Place the remaining cookies in a plastic bag and use a rolling pin to smash into crumbs.
15. Step
Arrange the sandwich cookies evenly around the top edge of the cake. Sprinkle the cookie crumbs over the exposed whipped cream.
