Preheat oven to 350° F. Grease a 9-inch fluted tart pan with a removable bottom and set aside.
For the brownies, melt butter in saucepan over moderate heat. Add sugar and stir to combine. Continue heating the mixture until it is hot but not bubbling; it will become shiny as you stir it.
Remove from heat and stir in cocoa powder and vanilla extract. Add eggs, one at a time, beating until smooth after each addition. Add salt and baking powder, stirring until combined. Add flour, ¼ cup at a time, stirring until the flour is incorporated.
Place prepared tart pan on a baking sheet. Scrape the thick, glossy batter into the pan, spreading it as evenly as you can. Scatter chocolate chips over the top.
Bake until the center is just set, about 22 to 25 minutes. Place pan on rack to cool.
To prepare the cinnamon pecans, toast pecans in a skillet over medium heat, stirring frequently until lightly toasted and fragrant. Add sugar and cinnamon and stir continuously until the sugar begins to melt. Continue cooking until pecans are shiny and well coated, about 5 minutes. Remove from heat and spread pecans in a single layer on a parchment-lined baking sheet (resist the temptation to use your fingers as they will be very hot). Cool completely.
For the caramel sauce, melt sugar over medium heat in a large saucepan, stirring constantly to ensure even heating. Once the sugar liquefies, reduce heat to low and continue cooking the sugar, stirring constantly, until it turns the color of a copper penny, about 2 to 3 minutes. Add butter and salt, stirring until fully incorporated. Remove from heat and add heavy cream, whisking vigorously until it forms a smooth sauce (the sauce will bubble and expand, so using a large saucepan will prevent boiling over at this stage). If the caramel separates from the cream, return to stove and heat until the sugar melts back into the cream.
Serve wedges of brownie topped with vanilla ice cream, warm caramel sauce, and chopped cinnamon pecans.