Sweet potato layer cake with spiced pecan buttercream

Name: Sweet potato layer cake with spiced pecan buttercream
Prep time: 1 h
Cook time: 1 h 30 min
Total time: 2 h 30 min
Cuisine: American
Ingredients: 250g pecan halves; 70g dark brown sugar; 2 tbsp ground cinnamon; 2 tsp ground nutmeg; 70g unsalted butter, melted; 2 tbsp dark rum; 900g sweet potatoes; 4 large eggs; 400g sugar; 250ml rapeseed oil; 280g cake flour; 2 tsp baking powder; 1 tsp salt; 1 tbsp ground cinnamon; 1 tsp ground nutmeg; 1 tsp ground ginger; 2 tsp pure vanilla extract; 3 tbsp rum; 125g chopped spiced pecans, recipe follows; 6 large egg yolks; 150g sugar; 150ml light golden syrup; 450g unsalted butter, room temperature; 1 tbsp vanilla extract; 1/8 tsp salt; 125g spiced pecans, recipe follows