Heat oven to 350ºF. In a food processor combine pretzels, ¼ cup brown sugar and butter; blend until mixture resembles fine crumbs. Press mixture firmly into bottom and up sides of a 9-inch pie pan. Bake crust until fragrant and just turning golden, 10 to 15 min; cool.
In a medium saucepan whisk together ½ cup brown sugar, cornstarch, cocoa, salt and milk. Place over medium heat and stir until sugar is just dissolved, about 2 min. Whisk in egg yolks. Cook, stirring constantly, until custard thickens and just boils, about 5 min. Remove from heat and stir in chocolate and vanilla. Place pan in a bowl of ice and water and stir occasionally until custard cools.
Pour custard into cooled crust and press plastic wrap directly onto surface. chill until firm, 4 hr or up to a day.
Whip cream and confectioners' sugar until soft peaks form, spread over pie and garnish with extra pretzels and chocolate shavings.
You can totally crush your pretzels in a plastic bag with a rolling pin, but I love blending them just right in a food processor like this one. I find it gets them to be just the right texture in a few pulses.