Make the dough: In the bowl of an electric mixer fitted with the paddle attachment, cream together butter, sugar, and salt on low speed until light, but not fluffy, scraping down sides of bowl as necessary. Add egg and mix well to combine. Add flour and mix until just combined. Turn out onto work surface and form into a disk; wrap with plastic wrap and transfer to refrigerator. Chill until firm, at least 1 hour.

Remove dough from refrigerator and let soften slightly. On a lightly floured work surface, roll out dough to an 11-inch circle, 1/8 to 1/4 inch thick. Fit dough into a 11-inch round tart pan and prick all over bottom with the tines of a fork. Transfer to refrigerator and chill until firm, about 45 minutes.

Transfer tart shell to oven; bake until golden, 30 to 35 minutes. Remove from oven and let cool completely.

Make the ganache: Place chocolate in a large heatproof bowl and set aside. In a medium saucepan, heat heavy cream over medium until small bubbles just begin to form around the edges. Pour heated cream over chocolate and let stand 5 minutes; add butter. Using an immersion blender or whisk, blend until smooth.

Pour ganache into cooled tart shell and transfer to refrigerator until set, at least 2 hours and up to overnight.

Make the meringue: Place egg whites, sugar, and salt in the heatproof bowl of an electric mixer set over (but not touching) simmering water. Cook, whisking, until mixture is warm to the touch.