Preheat oven to 350F. Grease 2 8-inch round pans, line the bottom with parchment paper, grease again then lightly dust with flour.
In a medium bowl, combine dry ingredients then set aside.
In a larger bowl using a handheld electric mixer or using a stand mixer fitted with the paddle attachment, beat butter and sugar on medium-high speed until light and fluffy.
Add the eggs one at a time, making sure each addition is incorporated.
Stir in the vanilla until combined.
Switch to low speed and add the flour mixture in thirds, alternating with buttermilk. Scrape the bottom and sides of your bowl as necessary.
Transfer the cake batter into your prepared pans and bake for 30 minutes or until a toothpick inserted in the centre of each cake comes out with minimal dry crumbs.
Allow the cakes to cool in the pan for 30 minutes before transferring to a wire rack to cool completely.
For the Raspberry Buttercream:
Using a handheld electric mixer or a stand mixer fitted with the paddle attachment, beat butter and raspberry jam on medium-high speed until smooth and creamy.
Switch to low speed and add confectioner's sugar a little at a time until well incorporated.
Once you've added all the sugar, add milk and beat the frosting on high speed for 2 minutes until very smooth and fluffy (see note).
For the Assembly:
Take one cake and place on a platter or cake turntable smooth side down.
Using a piping bag fitted with your favourite large tip, pipe a border around the cake.
Fill the centre with raspberry frosting. You'll want the height of the frosting equal to the height of your border so your cake doesn't sag in the middle.
Put your other cake on top, smooth side up and decorate as desired.
Chill in the fridge for 30 to 60 minutes to allow the frosting to set. Best served at room temperature.
Total yield depends on how thick or thin you slice your cake.
After adding the milk, check your frosting and decide if you're happy with the consistency. If not, add more sugar or milk about ½ teaspoon at a time and continue beating until you get the consistency you want. Be sure to taste the frosting as you go.
See post for lots of baking tips, FAQs and step-by-step photos.