Chocolate Cake with Milk Chocolate-Peanut Butter Frosting and Peanut Butter Brittle

Line large rimmed baking sheetwith foil; brush with oil. Bring sugar,corn syrup, and 1/4 cup water to boil inheavy medium saucepan over medium-highheat, stirring until sugar dissolves.Boil without stirring until syrup is deepamber, swirling pan occasionally, about10 minutes. Immediately stir in peanutsand all remaining ingredients. Scrape outmixture onto prepared sheet; spread out toabout 13x9-inch rectangle. Cool completely.Coarsely chop enough brittle to measure1 cup and finely chop enough to measure1/2 cup. DO AHEAD: Can be made 3 daysahead. Store in separate airtight containersat room temperature.

Preheat oven to 350°F. Spraytwo 9-inch-diameter cake pans with2-inch-high sides with nonstick spray.Line pan bottoms with parchment paper.Sift flour, sugar, cocoa, baking soda, andsalt into large bowl.

Whisk 2 cups water,oil, vinegar, and vanilla in medium bowlto blend. Gradually add water mixture todry ingredients, whisking until batter issmooth. Divide batter between preparedpans (scant 3 cups each).

Bake cakes until tester inserted intocenter comes out with a few crumbs stillattached, about 28 minutes. Cool cakes inpans 15 minutes.

Cut around cakes and turnout onto racks. Peel off parchment and cool.

Place chocolate in medium bowl. Bring 5tablespoons cream just to simmer in smallsaucepan over medium heat.

Pour cream overchocolate; whisk until melted and smooth.

Let stand until thick enough to spread,whisking occasionally, about 30 minutes.

Beat powdered sugar, 1 1/2 cups chilledcream, and peanut butter in large bowl justuntil blended.

Add mascarpone; beat frostingjust until thickened (do not overbeat).

Place 1 cake layer, flat side up, onplatter.

Spread milk chocolate filling evenlyover; sprinkle with 1/2 cup finely choppedpeanut brittle.

Spread with 1 cup frosting.Top with second cake layer, flat side down.

Spread remaining frosting over top andsides of cake. Cover with cake dome andchill 1 hour. DO AHEAD: Can be made 1day ahead. Keep chilled.

Let stand at roomtemperature 2 hours before continuing.

Sprinkle 1 cup coarsely chopped brittleover top of cake and serve.

* An Italian cream cheese; sold at manysupermarkets and at Italian markets.