1. Rub a terrine mold or loaf pan with oil and line with plastic wrap. You want enough wrap to hang over the edges like flaps. These should be long enough to cover the top of pan.
2. In a stand mixer, add egg whites and cream of tartar. Whisk until soft peaks form, then add sugar and beat on high for another minute until stiff but not dry. Reserve.
3. Add chocolate and butter to a microwave-safe bowl. Microwave for 30-second increments, stirring after each increment, until chocolate is melted. (You can also melt the chocolate and butter on a stove using a double boiler.)
4. Whisk in yolks and liqueur.
5. Gently fold chocolate into egg whites, being careful not to deflate the whites.
6. Once combined, scrape into the prepared loaf pan or terrine mold. Fold over plastic wrap flaps to cover. Refrigerate overnight.
7. When ready to serve, flip pan and gently remove marquise. Unwrap and slice with a warm knife into 4 to 6 portions. Top with crème anglaise (or melted vanilla ice cream, in a pinch) and some fresh berries.
