Preheat the oven to 350°F (175°C). Line a 9 x 13-inch pan with aluminum foil and spray the foil with nonstick cooking spray.
In a saucepan over low heat, melt the butter. Add the sugar and stir constantly until the mixture is hot to the touch (about 110–120°F) and looks shiny, but do not let it boil.
Meanwhile, crack the eggs into a mixing bowl and beat on low speed with an electric mixer until combined. Add the cocoa, salt, baking powder, espresso powder, and vanilla and mix until smooth and incorporated.
Pour the hot butter-and-sugar mixture into the egg-cocoa mixture and stir with a wooden spoon or spatula until smooth and combined.
Add the flour and stir just until combined; do not overmix.
Spoon half of the batter into the prepared pan and spread evenly. Arrange the Peanut Butter Snickers pieces in an even layer over the batter, gently pressing them slightly into the batter. Spoon the remaining batter over the candy pieces and smooth the top to cover them.
Bake for about 30 minutes, until the edges are set and the center appears very moist but not raw. Remove from the oven and cool completely on a rack before lifting from the pan, unwrapping the foil, and cutting into squares.