1. For the chocolate skull: Add the white chocolate melting wafers and coconut oil to a medium microwave-safe bowl. Microwave in 30-second intervals, stirring in between each, until melted and smooth, about 2 minutes.
2. Pour the chocolate mixture into the front and back cavities of a 2-part medium plastic skull mold (3-by-4-by-4-inch) and tilt to swirl the chocolate around until coated entirely (you can also use your finger to swirl the chocolate around). Tap out any excess and freeze until the chocolate is set, about 5 minutes.
3. Reheat the melted chocolate, if needed, and brush the sides of the mold cavities with a second layer. Freeze until set, about 5 minutes.
4. Carefully unmold the skull halves and fill one with the sliced strawberries. Microwave the melted chocolate again until smooth, about 30 seconds. Use the melted chocolate as glue to carefully attach the skull halves. Refrigerate until ready to serve.
5. For the cake and filling: Preheat the oven to 350 degrees F. Coat a 6-inch round cake pan with 3-inch sides with nonstick spray. Line the bottom of the pan with parchment and spray the paper.
6. Whisk together the flour, baking powder, baking soda and salt in a medium bowl.
7. Beat the butter in a large bowl with an electric mixer on medium speed until pale yellow, about 2 minutes. Add the confectioners’ sugar and beat on low just to incorporate, then beat on high until light and fluffy, 3 to 4 minutes. Add the vanilla and egg, scraping down the sides and bottom of the bowl, and continue to beat until creamy, about 2 minutes (the mixture will look curdled, but will come back together after everything is combined). With the mixer on low speed, add the flour mixture in 3 additions, alternating with the sour cream in 2 additions, beginning and ending with the dry ingredients.
8. Scrape the batter into the prepared pan and smooth the top.
9. Bake until golden on top and a toothpick inserted in the middle comes out clean, 35 to 40 minutes. Transfer to a rack and let cool 10 minutes, then run a knife around the edge of the pan, turn the cake out onto the rack, remove the parchment and cool completely.
10. Meanwhile toss the sliced strawberries and granulated sugar in a medium bowl. Set aside for at least 30 minutes to allow the strawberries to release their juices.
11. Cut the cake into 3 layers with a long serrated knife. Place a cake layer on a cake stand and top with half of the strawberries. Spread 3/4 cup of the whipped cream on top. Repeat with another cake layer, the remaining strawberries and 3/4 cup whipped cream. Top with the final cake layer and remaining 1/2 cup whipped cream. Position the skull in the center of the whipped cream.
12. For the strawberry white chocolate sauce: Add the strawberries and granulated sugar to a small saucepan over medium heat. Cook, stirring occasionally, until the strawberries have broken down and thickened slightly, 10 to 15 minutes. Remove from the heat and stir in the lemon juice and vanilla. Strain the mixture through a fine-mesh sieve and return the liquid to the saucepan (discard the solids). Add the cream and red candy melting wafers to the sauce and heat over medium heat, whisking constantly, until melted and hot, 10 to 15 minutes.
13. When ready to serve, pour the hot sauce over the center of the chocolate skull.