Line large rimmed baking sheet with foil; brush with oil. Bring sugar, corn syrup, and 1/4 cup water to boil in heavy medium saucepan over medium-high heat, stirring until sugar dissolves. Boil without stirring until syrup is deep amber, swirling pan occasionally, about 10 minutes. Immediately stir in peanuts and all remaining ingredients. Scrape out mixture onto prepared sheet; spread out to about 13x9-inch rectangle. Cool completely. Coarsely chop enough brittle to measure 1 cup and finely chop enough to measure 1/2 cup. DO AHEAD: Can be made 3 days ahead. Store in separate airtight containers at room temperature.
Preheat oven to 350°F Spray two 9-inch-diameter cake pans with 2-inch-high sides with nonstick spray. Line pan bottoms with parchment paper. Sift flour, sugar, cocoa, baking soda, and salt into large bowl. Whisk 2 cups water, oil, vinegar, and vanilla in medium bowl to blend. Gradually add water mixture to dry ingredients, whisking until batter is smooth. Divide batter between prepared pans (scant 3 cups each).
Bake cakes until tester inserted into center comes out with a few crumbs still attached, about 28 minutes. Cool cakes in pans 15 minutes. Cut around cakes and turn out onto racks. Peel off parchment and cool.
For filling and frosting:
Place chocolate in medium bowl. Bring 5 tablespoons cream just to simmer in small saucepan over medium heat. Pour cream over chocolate; whisk until melted and smooth. Let stand until thick enough to spread, whisking occasionally, about 30 minutes.
Beat powdered sugar, 1 1/2 cups chilled cream, and peanut butter in large bowl just until blended. Add mascarpone; beat frosting just until thickened (do not overbeat).
Place 1 cake layer, flat side up, on platter. Spread milk chocolate filling evenly over; sprinkle with 1/2 cup finely chopped peanut brittle. Spread with 1 cup frosting. Top with second cake layer, flat side down. Spread remaining frosting over top and sides of cake. Cover with cake dome and chill 1 hour. DO AHEAD: Can be made 1 day ahead. Keep chilled. Let stand at room temperature 2 hours before continuing.
Sprinkle 1 cup coarsely chopped brittle over top of cake and serve.