Preheat oven to 350 degrees. Grease and flour a bundt pan and set aside.
Make the Batter: Use stand mixer fitted with paddle attachment to cream butter until it is smooth, fluffy, and a slightly lighter shade of yellow (about thr minutes). Add the brown sugar and granulated sugar, and beat for another two minutes, then beat in the eggs, one at a time, followed by the milk, and vanilla extract, until uniform.
1 pound butter, 6 eggs, 1 cup brown sugar, 1 cup granulated sugar, ½ cup milk, 1 teaspoon vanilla extract
In another bowl, whisk together flour and sea salt. Add the wet mixture to flour mixture and stir by hand until batter is uniform and no dry pockets remain.
4 cups all-purpose flour, 1 teaspoon fine sea salt
Flavor the Batter: Divide batter evenly into two bowls. Mix the melted chocolate and cocoa powder into one bowl until uniform. Mix the peanut butter into the other bowl until uniform.
¼ cup semi-sweet chocolate chips, ¼ cup cocoa powder, ⅓ cup peanut butter
Create the Marble Effect: Fill prepared bundt pan about halfway with batter by adding alternating spoonfuls of chocolate and peanut butter batters. Use a butter knife to swirl the batters together then repeat process with remaining batter until pan is full.
Bake the Cake: Place pan in oven and bake for about an hour or until a toothpick inserted into multiple parts of the cake comes out clean. Remove cake from oven and let it sit in pan for about 10 minutes then carefully invert it onto a plate to finish cooling.
Prepare the Glaze: Use stand mixer fitted with the whisk attachment to beat the powdered sugar and butter together for about 30 seconds then add peanut butter and vanilla extract. Mixture will be somewhat coarse and crumbly.
1 cup powdered sugar, ¼ cup butter, ½ cup peanut butter, ½ teaspoon vanilla extract
Add in milk one tablespoon at a time until desired texture is reached then continue mixing for another three minutes or until the frosting is smoth and uniform. It should be glossy in appearance and thick enough that it doesn't run or easily drip off whisk when raised.
5 tablespoons milk
Decorate the Cake: Pour glaze over top of cooled pound cake and allow to fall down the sides. For an additional optional garnish, sift cocoa powder over top of cake.
cocoa powder