Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.

In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and light brown sugar with an electric mixer until light and fluffy, about 2-3 minutes.

Beat in the egg, vanilla extract, and 1 tablespoon of orange zest until well combined. Scrape down the sides of the bowl.

In a separate medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix. The dough will be thick.

Cover the dough with plastic wrap and chill in the refrigerator for at least 30 minutes. This helps prevent spreading.

Scoop rounded tablespoons of dough (about 1.5 inches in diameter) onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 10-12 minutes, or until the edges are set and the centers look slightly soft. The cookies will firm up as they cool.

Remove from oven and let cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.

In a clean large mixing bowl, beat the softened unsalted butter with an electric mixer until creamy and smooth, about 1-2 minutes.

Gradually add the sifted powdered sugar, 1 tablespoon of orange zest, fresh orange juice, vanilla extract, and a pinch of salt. Start on low speed to prevent sugar from flying, then increase to medium-high and beat until light, fluffy, and well combined, about 3-5 minutes. If the filling is too thick, add a tiny bit more orange juice (1/2 tsp at a time); if too thin, add more powdered sugar.

Once the cookies are completely cool, match them up into pairs of similar size.

Spread about 1-2 tablespoons of orange cream filling onto the flat side of one cookie.

Gently top with the flat side of the matching cookie, pressing lightly to create a sandwich.

Repeat with the remaining cookies and filling.