Preheat the oven to 350F/177C convection (325F/163C conventional) and line 13 muffin tins with cupcake liners (preferably a light colored pan).

In a medium bowl, whisk together the cake flour, baking powder, salt and sugar until they're evenly distributed (whisk well).

Add the oil and butter to the dry ingredients and beat together until they're combined.

Add the egg and vanilla extract and paste and mix until they're fully combined.

Add the sour cream and turn the mixer to low speed, while streaming the milk in. Scrape the edge of the bowl and mix just until the cupcake batter is smooth.

Place half of a cookie on the bottom of each cupcake liner and then fill the cupcake liner about half full with the vanilla batter (about 30-32 grams).

Place a teaspoon size chunk (5-7 grams) of cookie butter in the center of each cupcake and then top with just enough batter to cover the top (another 10-12 grams). The cupcake liners should be about ¾ full.

Bake for 20-21 minutes, just until the tops of the cupcake no longer look wet and when you press on the center of the cupcake it springs back.

Remove them from the oven and let them cool in the cupcake pan for 10 minutes, then move them to a cooling rack. Allow the cupcakes to cool completely before frosting them.

Place the sugar and water in a medium saucepan and swirl the pan until the sugar is fully immersed in water.

Place the saucepan over medium heat until the sugar dissolves and comes to a gentle simmer.

Reduce the heat to low and place the lid on the pan and allow the mixture to simmer for a couple minutes.

Meanwhile, place the eggs in the bowl of a stand mixer, fitted with the whisk attachment, and turn the mixer up to high speed. Let it whip while the sugar is cooking.

Use a candy thermometer to see when the sugar reaches 240F (116C), then lower the mixer to medium speed and slowly drizzle the hot sugar syrup into the thick and foamy eggs, while they are whipping.

Turn the mixer back up to high speed and let it whip for 5-10 minutes, until the mixture is very pale and fluffy and is cool to the touch or no longer warm.

Turn the mixer back down to medium speed and add in the butter, 2 Tbsp of butter at a time, mixing until each chunk is fully combined and the buttercream looks smooth and creamy.

Scrape the sides and bottom of the bowl, add in the vanilla, cookie butter and salt and turn the mixer up to high speed for 1-2 minutes. If the buttercream is too soft to frost, refrigerate it for 15-45 minutes.

Once the cupcakes have cooled, use a piping bag fit with a large round tip to frost the cupcakes.

Place the cookie butter in a small bowl and microwoave it for 10 seconds, or just until it's runny but not hot.

Use a spoon or a piping bag with a tiny round tip to drizzle the melted cookie butter onto the cupcakes and then top with crushed biscoff cookies.