Use cake mix to prepare 24 cupcakes according to package instructions. Let cool completely before decorating.

Meanwhile, in bowl of stand mixer fitted with paddle attachment, cream butter on medium speed until fluffy and smooth. Add strawberry jam and mix on medium speed until fully incorporated.

While beating on medium-low speed, add powdered sugar 1 cup at a time until all powdered sugar is incorporated.

Add vanilla and salt and mix on medium speed until incorporated and smooth. 

Using offset spatula, frost top of each cupcake thinly with prepared strawberry buttercream. This does not have to be perfect since this is just the base for the zombie brain frosting work you will do next. 

Place remaining buttercream in a piping bag fitted with a small round frosting tip. 

Pipe a line with a curve on the end (almost like an L shape) down the center of each cupcake. Then, pipe swirls and squiggles connecting from the end of that curve until you have covered the entire cupcake half. Repeat on the other side. 

Place cupcakes on rimmed serving tray and drizzle lightly with strawberry syrup.

The frosted cupcakes will keep for 3 days in an airtight container. I recommend drizzling the strawberry syrup / "blood" on just before serving.