In a microwave-safe bowl, combine the chopped chocolate and butter. Microwave in 30-second bursts, stirring between bursts, until melted and smooth. Set aside to cool slightly.

In the bowl of a stand mixer fitted with the whisk attachment, beat the sugar, eggs, and vanilla on medium-high speed for about 3 minutes, or until pale yellow and thickened (ribbon-like texture). Use a rubber spatula to fold in the slightly cooled melted chocolate mixture until combined. Sprinkle in the flour, baking powder, and salt. Stir gently with the spatula until just combined and no streaks of flour remain.

Chill the dough in the fridge for 1 hour —it will thicken and become mousse-like.

While the dough chills, preheat the oven to 350°F and line baking sheets with parchment paper.

Using a 1 ½-tablespoon cookie scoop, portion the dough onto prepared baking sheets, spacing the scoops about three inches apart. Note: The scoops might look uneven or messy, but they’ll spread and bake up beautifully.

Bake for about 9 minutes, until cookies are puffed, crackled, and no longer look wet in the centers. Let them cool for 5 minutes on the baking sheet. They’ll deflate slightly and become delicate, almost like a fudgy meringue. Gently use a thin spatula to transfer to a wire rack to cool completely.

Once cooled, transfer the cookies to a freezer-safe container and freeze until firm, at least 1 hour. Do not skip this step. Cookies can be stored in an airtight container in the freezer for up to 1 month.

Assemble the cookie sandwiches: Place one scoop of frozen cookie butter ice cream on the flat side of one brownie cookie. Top with another brownie cookie, flat side down, and gently press to create a sandwich. If desired, roll the edges of the sandwich in ground cinnamon for extra flavor and visual appeal.