How to Make It
Preheat oven to 375°F with oven racks in the top third and bottom third of oven. Beat butter and both sugars with an electric mixer on medium speed until smooth, about 1 minute. Add egg yolk and vanilla, and beat on low speed just until incorporated. Whisk together flour, salt, and baking powder in a small bowl, and gradually add to butter mixture, beating on low speed just until incorporated after each addition.
Drop dough by tablespoonfuls 1 inch apart onto parchment paper-lined baking sheets. Press your thumb or the end of a wooden spoon into each ball, forming an indentation.
Bake in preheated oven until cookies are set and beginning to brown, about 12 minutes, switching pans top rack to bottom rack halfway through baking. Cool on pans on wire racks 5 minutes; remove cookies to wire racks, and cool completely, about 30 minutes.
Fill each indentation with about 1/2 teaspoon jam, and let stand at room temperature 15 minutes.
Combine white chocolate and cream in a small heatproof bowl. Bring a small saucepan filled with 1 inch of water to a simmer over medium, and place bowl with chocolate mixture over simmering water. Cook, stirring often, until chocolate is melted and smooth, about 2 minutes. Cool 10 minutes. Spoon chocolate mixture into a ziplock plastic freezer bag with 1 corner snipped or a piping bag, and pipe mixture over cookies.