Snickers bars can be chopped small or larger to get large chunks of Snickers in every bite. Don't chop them too large though. I usually add 2-3 tablespoons of cocoa powder along with the flour, to deepen the chocolate flavor. I use Dutch-process cocoa powder, which makes the brownies extra dark.
In a large heat-proof bowl, place chocolate and butter. Heat in the microwave in 20-30 second intervals, stirring in between each interval, until melted (or set it over a saucepan of simmering water, stirring occasionally). Add sugar to chocolate mixture and whisk until combined. Whisk in eggs, one at a time. Add flour and salt and fold, using a rubber spatula, just until combined. Do not overmix. Fold in Snickers pieces.
Pour batter into prepared pan and smooth the top. Bake for 18-22 minutes, until the edges are set but the center is still very soft. Be careful not to overbake. A toothpick inserted into the center of the brownies should come out with moist crumbs, not dry. Let brownies cool completely on a wire rack, then refrigerate until set for easy cutting. Lift brownies out of the pan using parchment paper and cut into bars.
Store brownies at room temperature or in the fridge in an airtight container for up to 4 days. Brownies can be frozen for up to 2 months. Thaw them overnight in the fridge.