Preheat the oven to 350F/177C convection (or 325F/162C conventional) and use baking spray to grease three 6 inch cake pans.

In a medium bowl, whisk together the flour, baking powder, salt and cinnamon. Set aside.

Place the oil and peanut butter in a large bowl and use an electric mixer at medium speed, to whisk until it becomes smooth.

Add brown sugar and granulated sugar to the mixture and beat for 1-2 minutes.

Scrape the edges of the bowl and add in the eggs, one at time. Beat on medium speed until each one is fully incorporated, making sure to scrape the edge of the bowl as needed.

With the mixer on low speed, add in 1/2 of the dry ingredients, followed by the buttermilk and the other half of the dry ingredients. Mix just until the last few streaks of flour are combined.

Divide evenly among the prepared pans and bake for 30-35 minutes, until the edges release from the pan or a toothpick inserted in the center comes out clean.*

Flip the cakes over onto a towel to remove from the cake pan and allow them to cool completely.

In a small saucepan over medium-low heat, bring the cream to a gentle simmer.

Place the chocolate chips in a bowl and pour the hot cream on top. Cover the bowl and let it sit for 1-2 minutes then stir until it's smooth. Allow the chocolate to cool before using.

In the bowl of a stand mixer, beat the butter until it's pale and fluffy.

Add the powdered sugar, cocoa powder and salt and beat for a minute on low speed until it's mostly combined. Increase the speed to medium-high and beat for 5-10 minutes, until the sugar feels fully dissolved.

Add in the melted (cooled) chocolate and vanilla extract, beating until they're fully combined.

Switch to a paddle attachment and cream the buttercream for 2-3 minutes on low-medium speed.

Once the cake layers have cooled completely, trim the tops of the cake to be flat.

Chop up Reese's peanut butter cups into quarters, or smaller.

In a heat proof bowl, microwave the peanut butter for 15 seconds, until it's melted but not burned.

Use 1/4 of the buttercream to frost the first cake layer, then top it with a drizzle of melted peanut butter and a handful of chopped peanut butter cups. Repeat with the second layer.

Place the third layer on top and use the other 1/4 of the buttercream to make a crumb coat. (A crumb coat is a super thin layer of frosting to keep the crumbs in). Refrigerate the cake for at least 30 minutes.

Remove the cake from the refrigerator and use the rest of the buttercream to neatly frost the cake (you may have to whip the buttercream a little bit again).

Top the cake with the rest of the melted peanut butter (you may have to reheat it) and the Reese's peanut butter cups.