Make the chewy double-chocolate cookies: Preheat the oven to 325° and line 2 baking sheets with parchment paper.
In a medium bowl, whisk together the flour, cocoa powder, baking soda and kosher salt. Set aside.
Pour the melted butter into the bowl of a stand mixer fitted with a paddle attachment. Gradually add both sugars, beating on medium-high speed, until a smooth and sticky mixture forms, about 4 minutes. Reduce the speed to low and add the egg, egg yolk and vanilla. Add the dry ingredients and mix until just barely incorporated. Remove the bowl from the stand mixer and stir in the chocolate chips.
Take ¼ cup of the cookie dough and, using your hands, gently roll it into a ball and transfer to your prepared baking sheet. Flatten the ball with the palm of your hand to measure 2½ inches. Repeat with remaining dough and sprinkle a little flaky sea salt on top of each cookie. Bake the cookies until the edges begin to crack, 10 to 11 minutes. Makes 12 cookies.
Make the chocolate glaze: In a small saucepan, combine the cream, maple syrup and cocoa powder, and bring to a simmer over medium-low heat. Add the chocolate chips and continue to cook, stirring constantly, for 1 minute. Remove from the heat, then stir in the vanilla and salt. Set aside and let cool to room temperature. Makes about ½ cup.
Poach the dried mango: Arrange the dried mango pieces in one layer across the bottom of a medium saucepan. Pour in the orange juice and water, and place the saucepan over low heat. Gently bring the mangoes to a low simmer and cook until the mangoes are soft, 3 to 5 minutes. Discard the poaching liquid and set the mangoes aside to cool.
Assemble the ice cream sandwiches: Spread 2 teaspoons of chocolate glaze on the bottom of each cookie. Using your hands, scatter sprinkles onto the outer ½-inch rim of the glaze. Lay a piece of the mango on top. Transfer the open-faced cookies to a baking sheet and chill in the fridge until the glaze is firm, 12 to 15 minutes.
Scoop the Tillamook Mudslide ice cream onto a mango cookie and top with another cookie, gently pressing to sandwich together. Repeat with the remaining cookies and ice cream. Serve immediately or store in the freezer until ready to serve.