Step 1

Preheat oven to 350°F. Line a standard 12-cup muffin tin with paper liners. In a large bowl, whisk together sugar, flour, cocoa, baking soda, baking powder, and 3/4 teaspoon salt. Add buttermilk, 1/3 cup plus 1 tablespoon oil, and warm water; whisk to combine.

Step 2

Add eggs, one at a time, then 1 teaspoon vanilla, whisking until smooth. Divide batter evenly among prepared cups, filling each about two-thirds full. Bake, rotating tin halfway through, until tops spring back when lightly touched and a tester comes out with a few moist crumbs attached, 20 to 25 minutes.

Step 3

Transfer tin to a wire rack; let cool 10 minutes. Turn out cupcakes onto rack; let cool completely.

Step 4

Divide buttercream in half. Stir peanut butter and remaining 1/4 teaspoon salt into one half, and place in a pastry bag with no tip. Stir remaining 1 teaspoon vanilla into other half, and place in another pastry bag with no tip. Fit a third pastry bag with a large round tip, about 1/2-inch diameter. Snip ends off both bags of buttercream; insert them in pastry bag with tip. Squeeze gently until both buttercreams come out evenly. Pipe a swirl of buttercreams onto each cupcake, about 2 inches high. Refrigerate at least 25 minutes.

Step 5

In a heatproof bowl set over (not in) a pan of simmering water, melt chocolate with remaining 3 tablespoons oil, stirring until smooth. Remove from heat; let cool slightly. Transfer to a tall, wide container, such as a quart takeout container, and let cool completely. (Glaze should be smooth but not warm.)

Step 6

Gently dip cupcakes into glaze to coat tops completely, holding above container to allow excess glaze to drip off. Return cupcakes to refrigerator to set, about 10 minutes, before serving. Finished cupcakes can be refrigerated in an airtight container up to 3 days.