One of my favorite cakes is raspberry lemonade. I make it with 4 layers of lemon cake filled with 2 layers of raspberry filling and one layer of lemon curd. It is delicious, and so pretty when cut. It is usually frosted with lemon buttercream.
After much trial and error, I found the Cake Boss raspberry filling to be the best. It is full of flavor, but not thin and runny. Even so, a thin layer is plenty, and I always put a dam of buttercream around the outside of fruit fillings and curds to prevent oozing out the sides. (I use RLB's lemon curd recipe). I think it is OK to post the raspberry filling recipe as it is widely available on the internet (see below). If you wish to have a lighter filling, or to make lemon or raspberry frosting, mix lemon curd or raspberry filling into SMBC or IMBC, approximately 1/4 cup per 2 cups of buttercream, adjusting to taste.
You can put a layer of filling on top of a layer of buttercream as you mentioned above, but that makes the layers more prone to slip and slide. If you want to use a fruit filling and buttercream as a filling, I might suggest alternating ie: cake, filling, cake , buttercream, cake, filling, cake.
Source: Cake Boss
Notes: yield 1 cup
keeps 1 week refrigerated
keeps frozen at least 3 months
12 ounce bag of frozen unsweetened raspberries
½ cup sugar
1 tsp lemon juice
2 heaping tablespoons cornstarch
2 tablespoons raspberry liqueur like Chambord (optional)
Thaw raspberries in a strainer, collecting juice. This will take several hours or overnight. Add enough water to make ¾ cup liquid. Set aside the drained berries. In a small saucepan combine raspberry and lemon juices, sugar and cornstarch. Cook over medium heat stirring constantly until mixture thickens and bubbles. Remove from heat. Strain remaining berries to remove seeds. Add pulp and liqueur to cooled mixture in saucepan. Stir well and refrigerate for several hours until completely cool.