Preheat your oven to 350°F (175°C) and line a 9×13 inch baking pan with parchment paper, leaving overhang on the sides for easy brownie removal.
Melt the butter in a large microwave-safe bowl in 30-second intervals, stirring between each until completely melted but not hot.
Whisk sugar into the melted butter until well combined, then add eggs one at a time, whisking well after each addition until the mixture becomes slightly lighter in color.
Stir in vanilla extract until fully incorporated into the wet mixture.
Sift together flour, cocoa powder, and salt in a separate bowl to remove any lumps and ensure even distribution of ingredients.
Gently fold the dry ingredients into the wet mixture using a spatula, being careful not to overmix—stop when you no longer see dry streaks.
Fold in the chocolate chips until evenly distributed throughout the batter.
Pour the batter into your prepared pan and spread evenly using a spatula or the back of a spoon.
Bake for 25-30 minutes until a toothpick inserted comes out with a few moist crumbs (not wet batter). Be careful not to overbake—fudgy brownies are key!
Allow brownies to cool completely in the pan, then use the parchment overhang to lift them out and cut into 2-inch squares.
Set up your sundae bar by arranging brownies on a platter and placing all toppings in separate bowls with appropriate serving utensils.
Keep ice cream in the freezer until just before serving, then place in an ice bath to maintain consistency while guests build their sundaes.
Warm sauce containers by placing them in warm (not hot) water for a few minutes before serving.