Passion fruit curd is spooned into the center of these cupcakes, infusing them with a sweet surprise. Martha made this recipe on Martha Bakes episode 601.
Make the passion fruit curd: In a medium saucepan, whisk together sugar and egg yolks; whisk in passion fruit puree and salt. Add butter and place pan over medium-high heat. Cook, whisking constantly, until butter has melted, mixture is thick enough to coat the back of a spoon, and small bubbles form around the edge of the pan, about 5 minutes.
Remove pan from heat while continuing to whisk. Strain curd through a fine-mesh sieve into a bowl. Cover with plastic wrap, pressing directly onto the surface of the curd to prevent a skin from forming. Refrigerate until chilled, at least 2 hours. Rinse and dry saucepan.
Make the cupcakes: Preheat oven to 350 degrees. Line two standard 12-cup muffin pans with paper liners. In a medium bowl, sift together flours, baking powder, and salt.
In the bowl of an electric mixer fitted with the paddle attachment, beat sugar and butter, scraping down the sides as needed, until light and fluffy, 3 to 4 minutes. Beat in eggs, one at a time, then beat in vanilla. With the mixer on low speed, add flour mixture in three additions, alternating with the milk and beginning and ending with the flour mixture; beat until combined after each addition.
Divide the batter evenly between the prepared cups, filling each about two-thirds full. Bake, rotating pans halfway through, until cupcakes are golden brown and a cake tester inserted in a center comes out clean, about 20 minutes. Transfer pans to a wire rack to cool. Invert cupcakes onto the rack; reinvert and let cool completely.
Make the meringue buttercream: While cupcakes are baking, fill the reserved saucepan with about 2 inches water and bring to a simmer. Combine sugar, egg whites, and salt in the heatproof bowl of a standing mixer. Set bowl over (not in) simmering water, and whisk until mixture is warm to the touch and sugar dissolves, 2 to 3 minutes. (Test by rubbing between your fingers; the mixture should feel smooth.)
Attach bowl to standing mixer fitted with whisk attachment, and beat on low speed until foamy. Add cream of tartar, and beat on medium-high speed until stiff, glossy peaks form and mixture is cooled completely, about 10 minutes.
Reduce speed to medium-low; add butter, 2 tablespoons at a time, beating to incorporate fully after each addition. (Buttercream may appear curdled at this point; it will become smooth again with continued beating.) Beat in vanilla. Switch to paddle attachment. Beat on lowest speed, 3 to 5 minutes. Transfer to a pastry bag fitted with a large star tip.
Assemble the cupcakes: Using an apple corer, make a small hole in the top of each cupcake, reserving tops. Fill each cupcake with about 1 tablespoon chilled passion fruit curd. Replace reserved tops, then pipe frosting onto cupcakes, as desired.
Store the cupcakes covered in the refrigerator for up to 3 days; let stand at room temperature for 10 minutes before serving.