Make the Cookies:

1. Preheat the Oven: Set to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats.
2. Combine Dry Ingredients: In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
3. Cream Butter and Sugar: Using an electric mixer, beat butter and sugar on medium-high speed until pale and fluffy (about 3 minutes).
4. Add Wet Ingredients: Mix in egg white, heavy cream, and vanilla extract until well combined.
5. Incorporate Dry Ingredients: Reduce mixer speed to low, adding the flour mixture in three additions. Mix until fully incorporated, scraping the bowl as needed.
6. Shape the Dough: Scoop 1 heaping tablespoon of dough, roll into balls, and place 2 inches apart on prepared baking sheets. Flatten each ball to about ¼ inch thick and 2 inches in diameter using moistened fingers.
7. Bake: Bake one sheet at a time for 8-10 minutes, rotating halfway. Cookies are ready when edges start to brown.
8. Cool: Let cookies cool completely on the baking sheets.

2. Make the Frosting:

1. Melt Chocolate: Combine chocolate chips, heavy cream, and salt in a microwave-safe bowl. Microwave at 50% power, stirring occasionally, until smooth (1-3 minutes).
2. Mix in Dry Ingredients: Whisk in cocoa powder, powdered sugar, and vanilla extract until the frosting resembles thick brownie batter (around 95°F).

3. Assemble the Cookies:

1. Prepare for Frosting: Place cooled cookies flat-side up on the baking sheet.
2. Add Frosting: Spoon about 2 tablespoons of frosting onto each cookie, creating a smooth mound.
3. Set Frosting: Let cookies sit at room temperature for about 3 hours until the frosting firms up.