This sinfully delicious cake is a killer combination of my beloved rich Chocolate Cake smothered with fluffy peanut butter frosting, topped with Reece’s Peanut Butter Cups then finished with a drizzle of melted chocolate and peanut butter.

It’s decadent and outrageous, so it’s perfect for feeding a crowd to a) impress b) it will easily serve 20 to 25 people, up to 30 – you don’t need giant slices because it’s so rich!

You’ll love how easy the chocolate cake is to make, and the touch of salt in the frosting.

Ingredients

1 3/4 cups flour , plain / all purpose
3/4 cup cocoa powder , unsweetened (Note 1)
1 1/2 tsp baking powder
1 1/2 tsp baking soda (bi-carb soda) (Note 2)
2 cups white sugar or caster/superfine sugar
1 tsp cooking / kosher salt (yes, really!)
2 large eggs (~55-65g / 2 oz each – see here for size info)
1 cup milk (low or full fat)
1/2 cup vegetable oil , or other plain flavoured oil like canola, grapeseed, rapeseed, sunflower
2 tsp vanilla extract
2 tsp instant coffee powder , OPTIONAL (Note 3)
1 cup boiling water (makes the chocolate flavour “bloom”)
Peanut Butter Buttercream Frosting
250g / 2 sticks unsalted butter , softened
4 cups (500 g) soft icing sugar / powdered sugar , sifted (Note 4)
1/2 cup (120 g) smooth peanut butter (spread, not pure) (Note 5)
1/4 tsp cooking/kosher salt
1 tbsp milk
Toppings (optional, recommended)
1/2 cup chocolate chips or melts (US: semi sweet chips)
2 tbsp cream (thickened/heavy or pure, not low fat)
3 tbsp peanut butter , warmed in microwave to make it runny
120g / 4 oz mini Reece’s Peanut Butter Cups
10+ miniature Reece’s Peanut Butter Cups , halved
1/4 cup salted peanuts , finely chopped

Instructions

Preheat oven to 180C°/350°F (160°C fan).
Grease a 23 x 33cm / 9 x 13″ metal cake pan with butter. Then line with paper with overhang, to make it easier to lift out. (For round pans, see Note 4).
Batter:
Dry ingredients – Sift flour, cocoa, baking powder and baking soda into a large bowl. Add Sugar and salt. Whisk briefly to combine.
Wet ingredients – Add eggs, milk, oil and vanilla. Whisk well to combine until lump free – about 30 seconds.
Boiling water – Add the coffee powder then the boiling water. Whisk to incorporate. The batter is VERY thin (see video). Pour batter into the cake pan.
Bake:
Bake for 45 minutes or until a wooden skewer inserted into the centre comes out clean.
Cool for 10 minutes, then fully cool on a wire rack before frosting (~2 hours).
Peanut butter buttercream frosting
Beat butter for 3 minutes on high until soft and fluffy, using a stand mixer with the paddle attachment or handheld beater.
Icing sugar – Add icing sugar in 3 lots. Beat it in, starting on low speed then increasing the speed (to avoid a snow storm). Once incorporated, add more icing sugar and repeat.
Beat 3 minutes – Once the icing sugar has all been added, added the peanut butter, salt and milk. Beat on high speed for 3 minutes to make it really nice and fluffy. Ready to use!
Decorating
Chocolate drizzle – Place the chocolate and cream in a small bowl. Microwave in 15 second increments on high, stirring in between, until smooth. Cool for 10 minutes before using.
Frost – Spread all the frosting across the surface of the cake, from edge to edge.
Decorate – Drizzle the top randomly with half the chocolate drizzle and melted peanut butter. Sprinkle with the peanut butter cups. Drizzle with remaining chocolate and peanut butter and sprinkle with the peanuts.
Serve immediately! Cut into pieces as large (or small 😢) as you want. Refrigerate leftovers for 5 days though always bring to room temperature (because the frosting goes hard when cold).

Notes

2. Cocoa powder – I use regular cocoa powder though you could use dutch processed for a slightly more intense chocolate flavour (not that you need more!).
2. Baking soda aka bi-carbonate soda, makes cakes etc rise like baking powder but it’s ~3x stronger. They work differently in different batters, and for this one a combination of both makes the most tender cake with a near level surface. Substitute with an extra 4 1/2 teaspoons of baking powder.
3. Instant coffee powder – Optional but recommended! This unusual sounding ingredient in a chocolate cake is a little baking trick: a touch of coffee enhances the flavour of chocolate but you won’t taste coffee flavour at all.
Be sure to use instant coffee powder (ie the type that dissolves in hot water) NOT ground coffee beans that is used for plungers and coffee machines (it doesn’t dissolve in hot water). Substitute with hot freshly brewed coffee instead of boiling water.
4. Australia – Be sure to get SOFT icing sugar, not PURE icing sugar (that’s for hard icing like Royal Icing). If you’re outside Australia, you don’t need to worry about this.
5. Peanut butter spread – I use peanut butter spread here (like Bega, Jiffy’s etc) which has salt and sugar added. If using natural unsweetened peanut butter, reduce to 6 tbsp (90g) else your frosting may be too loose (because it’s runnier).