½ cup (1 stick) unsalted butter (plus more for greasing the pan)
3 large eggs
1 teaspoon vanilla extract
2 cups granulated sugar
¾ cup all-purpose flour
To make caramel, heat cream with cayenne in small saucepan set over medium heat until small bubbles form. Set aside and keep hot.
In heavy-bottomed pot, melt sugar over medium-high heat. Use paring knife to scrape down sugar from sides of pot, if necessary. Once sugar begins to melt, turn heat down to low and continue melting. Stir only to break up sugar.
Once all sugar is dissolved and caramel is medium-amber in color, slowly pour spicy cream into caramel, whisking until fully incorporated. (Should sugar re-crystallize, do not panic—keep stirring and it will re-melt.) To ensure there are no remaining clumps of sugar, strain caramel before proceeding.
Add peanut butter and stir until smooth. Remove from heat and stir in butter. Allow to cool in refrigerator for 20 minutes.
To make brownie batter, melt chocolate with stick of butter in small saucepan set over medium heat, stirring frequently until completely melted; take care not to scorch. Remove from heat and cool.
In standing mixer, beat eggs until light, frothy, and even in color. Beat in salt and vanilla.
Temper eggs by adding small amount of melted chocolate with mixer on medium. Add remaining chocolate to eggs, then add sugar.
Turn mixer to low and slowly add flour until just incorporated.
To assemble brownies, preheat oven to 375 degrees. Grease 9x13-inch baking pan with remaining butter. Pour in half of brownie batter, tilting pan to level it.
With tablespoon, spoon half the caramel on top of batter in two even rows, two inches apart. Pour remaining batter into pan and spoon remaining caramel on top. Drag tip of knife through batter to create marble effect.
Bake for 35-40 minutes, or until toothpick inserted in center comes out clean.
Remove from oven and let cool completely before cutting.