Make the purée
Put the strawberries in a food processor (a blender also works) and whiz until completely smooth, 1 to 2 minutes. Pour the purée into a medium saucepan and bring to a boil over medium-high heat, stirring occasionally. Reduce the heat to low and cook, stirring often, until very thick and reduced to 3/4 cup, 18 to 22 minutes. Slide the pan from the heat and let cool completely.
Make the cupcakes
Position an oven rack in the center of the oven and heat oven to 375°F. Line 12 regular-sized (2-3/4-inch diameter) muffin cups with paper or foil liners. Put the flour, sugar, baking powder, and salt in a medium bowl and whisk until well blended.
Put the melted butter, water, egg, egg yolk, and vanilla in a small bowl and whisk until well blended. Pour over the dry ingredients and whisk until well blended and smooth, about 1 minute.
Portion the batter evenly among the prepared muffin cups (3 Tbs. batter per cup will be about two-thirds of the way full). Bake until a toothpick inserted in the center comes out clean, 14 to 16 minutes. Move to a wire rack and let cool for 15 minutes. Carefully remove the cupcakes from the pan, set them on a wire rack, and let cool completely.
Make the frosting
Put the heavy cream and mascarpone in a large bowl. Beat with an electric mixer fitted with the whip attachment on medium-low speed until well blended and smooth, about 1 minute. Add the confectioners’ sugar, vanilla, and salt. Beat on medium-high speed until thick enough to hold firm peaks when the beater is lifted, about 2 minutes. Add 3/4 cup of the cooled, reduced strawberry purée and beat on medium until well blended.
Assemble the cupcakes
Portion the frosting evenly among the cooled cupcakes (about 2 Tbs. per cupcake). Using a small metal spatula, mound and swirl the frosting on top of the cupcakes, leaving a border of cake around each one.
Top each cupcake with a small strawberry. Serve immediately or chill for 30 minutes.
Make Ahead Tips
The cupcakes are best when frosted and eaten the day they’re baked. Serve slightly chilled but not cold.
The condensed strawberry purée can be made, covered, and refrigerated for up to 2 days before using.
The strawberry cream frosting can be made, covered, and refrigerated up to 1 day ahead.
Photo: Miki Duisterhof