In a stand mixer fitted with the paddle attachment, beat the butter, 3/4 cup pretzels and the confectioners’ sugar on low until creamy. Beat in the flour, egg and the remaining 1/2 cup pretzels making sure to leave some pretzel pieces. Between two sheets of plastic wrap, pat the dough into a disc. Chill in the refrigerator for at least 30 minutes.
Preheat the oven to 350 degrees F. Roll out the dough between the plastic wrap sheets into a 12-inch circle. Remove the top piece of plastic wrap and invert the dough into a 10-inch fluted tart pan. Press the dough into the sides of the pan patching any tears as necessary. Remove the other sheet of plastic wrap. Trim the excess dough and chill in the refrigerator for 30 minutes or until firm.
Line the shell with parchment and fill with pie weights. Bake for 30 minutes until the dough is almost cooked through. Remove the parchment and weights and continue cooking until set, an additional 10-15 minutes. Cool the shell completely before brushing with the melted bittersweet chocolate. Chill in the refrigerator for 10 minutes.
Place the milk chocolate in a large bowl. In a small saucepan over medium heat, bring the heavy cream to a simmer. Pour the cream over the chocolate and let stand for 5 minutes before gently whisking until smooth. Cool to room temperature, about 1 hour.
Pour the milk chocolate filling into the shell and chill in the refrigerator until set, at least 4 hours. Sprinkle with sea salt and pretzels before serving.