2/3 cup (150 grams) unsalted butter, softened1 cup (200 grams) granulated sugar2 large eggs (100 grams), room temperature1 teaspoon (4 grams) vanilla extract1¼ cups (156 grams) all-purpose flour1¼ teaspoons (6.25 grams) baking powder3/4 teaspoon (2.25 grams) kosher salt1/3 cup (80 grams) whole Greek yogurt, room temperature1 tablespoon (15 grams) whole milk1 ounce (28 grams) bittersweet chocolate, melted and cooled slightly1 tablespoon (4 grams) instant espresso powderWhipped Mascarpone (recipe follows)Unsweetened cocoa powder, for sifting
Preheat oven to 350°F (180°C).
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
In a medium bowl, whisk together flour, baking powder, and salt. In a small bowl, whisk together yogurt and milk until combined. With mixer on low speed, add flour mixture to butter mixture in three additions alternately with yogurt mixture, beginning and ending with flour mixture, beating just until combined after each addition.
In another medium bowl, whisk together melted chocolate and espresso powder until espresso fully dissolves. Stir in half of batter.
Spray a 6-cup charlotte cake pan with baking spray with flour. Spoon plain and chocolate batters in an alternating pattern into prepared pan. Firmly tap pan on a kitchen towel-lined counter to settle batter. Using a wooden pick or skewer, swirl batters together.
Bake until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Let cool in pan for 10 minutes. Invert cake onto a wire rack, and let cool completely.
Just before serving, spread Whipped Mascarpone onto cooled cake, and sift cocoa on top. Refrigerate in an airtight container for up to 3 days.