MAKE THE CRUST: In a standing electric mixer fitted with the paddle, beat the butter with 3/4 cup of the pretzels and the confectioners' sugar at low speed until creamy. Beat in the flour and egg. Add the remaining 1/2 cup of pretzels, being sure to leave some pretzel pieces intact. Flatten the dough between 2 sheets of plastic wrap and refrigerate until chilled, at least 30 minutes.
Preheat the oven to 350°. Roll out the dough between the sheets of plastic wrap to a 12-inch round. Peel off the top sheet and invert the dough over a 10-inch fluted tart pan with a removable bottom. Press the dough into the corners and patch any tears. Trim the overhanging dough and refrigerate the shell for 30 minutes or until firm.
Line the shell with parchment paper and fill with pie weights. Bake for about 30 minutes, until nearly set. Remove the parchment and weights and bake for 10 to 15 minutes longer, until the tart shell is firm; cover the edge with foil if it darkens too much. Let the shell cool completely. Brush the melted chocolate over the bottom and up the side and refrigerate for 10 minutes, until set.
MEANWHILE, MAKE THE FILLING: In a medium saucepan, bring the cream to a simmer. Off the heat, add the milk chocolate and let stand for 5 minutes. Whisk until smooth. Transfer the filling to a bowl and let cool to room temperature, about 1 hour.
Pour the filling into the shell and refrigerate until set, at least 4 hours. Sprinkle lightly with sea salt and crushed pretzels. Cut into wedges, top with crème fraîche; serve.