Preheat oven to 350°F (180°C).: The kitchen will fill with a warm, faintly sweet scent as the oven reaches temperature, signaling the start of the baking rhythm. Preheating ensures an even rise and proper bake from the first minute, so do not skip this, and avoid opening the door while it warms. A common error is placing batter into an oven that is still cold, which can yield uneven texture and poor lift.
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.: You will notice the unsalted butter and granulated sugar transform from glossy to pale and aerated, a sign that air has been incorporated to create lift. The sound changes from a low churn to a lighter, airy whir as bubbles form. Adding the eggs slowly keeps the emulsion stable and prevents curdling. If the mixture looks separated after an egg, pause and scrape the bowl, then continue; rushing is the usual mistake here.
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.: The dry all-purpose flour blend should be light and lump free, and the small bowl of whole Greek yogurt with whole milk will be silky. Alternating additions keeps the batter smooth and prevents overworking gluten, preserving tenderness. You will see the batter go from glossy to slightly thicker; stop mixing as soon as streaks of flour disappear. Overmixing at this stage creates a tougher crumb, so resist the urge to keep stirring.
In another medium bowl, whisk together melted chocolate and espresso powder until espresso fully dissolves. Stir in half of batter.: As you combine the warm, melted bittersweet chocolate with the instant espresso powder , a rich aroma of coffee and cocoa will rise, evoking that tiramisu character. Stirring this into half the batter creates the marbled effect and pockets of intense flavor. Ensure the chocolate is only slightly warm to avoid loosening the batter too much. A potential pitfall is adding hot chocolate directly, which can collapse the incorporated air.
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.: The prepared pan will feel cool to the touch, and as you spoon alternating dollops of plain and chocolate batter, the contrast is visually appealing. Tapping the pan releases trapped air pockets and helps the batter level. Swirling with a skewer should create loose marbling rather than intricate knots, which gives varied bites. Avoid over swirling which blends the batters into a uniform color and loses the desired contrast.
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.: While baking you will notice the edges pulling slightly from the pan and the top turning golden; that is your signal the interior is setting. The center should spring back gently to the touch. Cooling in the pan for 10 minutes stabilizes the structure, then inverting onto a wire rack helps moisture escape so the bottom does not become soggy. Cutting the cake while too warm is the frequent misstep, leading to a crumbly slice, so be patient.
Just before serving, spread Whipped Mascarpone onto cooled cake, and sift cocoa on top. Refrigerate in an airtight container for up to 3 days.: The final touch is the airy Whipped Mascarpone , which when spread creates a cool, creamy contrast to the warm chocolate aroma already present. Sifting the unsweetened cocoa powder over the top adds a dry, slightly bitter note that balances richness. Chilling melds flavors and firms the topping, but leaving it out too long can make the mascarpone weep, so keep it refrigerated until serving. A common error is spreading mascarpone on a warm cake which causes sliding and imbalance in texture.