For the cardamom white chocolate coating: Melt the white chocolate and coconut oil in the microwave. Stir in the ground cardamom. Set aside at room temperature until needed.
To assemble: Transfer half of the melted chocolate to a small piping bag and snip the tip so that the opening is about 1/4-inch-wide (or fit the bag with a 1/4-inch tip).
Line a baking sheet with parchment paper. Place half of the cookies upside down on the prepared baking sheet. Pipe a circle of the white chocolate in the middle of the cookies. Sandwich with the remaining cookies and press very gently together. Freeze for 15 minutes to set.
Make sure the white chocolate is smooth and melted by reheating if necessary. Dip half of the cookie sandwich in the remaining white chocolate and allow the excess to drip off.
Place the dipped cookie sandwich back on the baking sheet and freeze for 15 minutes to set.
Brush the white chocolate with the colored powder if using. Keep refrigerated until ready to serve.