These dense treats will be snapped up off your dessert buffet.
Heat bittersweet chocolate, cream, and espresso powder, in a small saucepan over low heat, stirring, just until chocolate melts and mixture is smooth. Add 1 heaping tablespoon chocolate mixture to each tartlet shell.
Melt white chocolate, stirring, in a heatproof bowl set over a saucepan of simmering water. Transfer to a resealable plastic bag; snip a small opening in the corner. Pipe 4 dots of the melted white chocolate on filling in each tartlet. Using the tip of a knife, swirl filling to marbleize. Gently tap on work surface to settle filling. Chill tartlets until firm, at least 1 hour.