Prepare a 7" springform pan, lining the bottom with parchment paper and lightly spraying the sides with a cooking spray that has added flour.
Sift the flour and baking powder into a medium-sized bowl and set aside.
Beat the butter and sugar together with an electric or stand mixer on medium-high until light and fluffy.
Beat in the vanilla.
Reduce mixer speed to low and add the flour mix alternately with the milk, mixing until just incorporated. Increase speed to medium-high and beat for 2 minutes.
Pour the batter in the prepared pan, lightly tapping on the counter a few times to remove any air pockets.
Bake for 30-40 minutes or until a toothpick inserted in the middle comes out clean and the cake springs back to the touch.
Cool the cake in the pan for 10 minutes before turning out on a wire rack to cool completely.
Make the Buttercream
While the cake is cooling, cream butter and salt, mixing until well combined and smooth
At medium-slow speed, add powdered sugar ¼ cup at a time until smooth
Add in water and extract, increase speed to high and beat at least 5-6 minutes. If the buttercream is too thick, add in additional water ½ tsp at a time until desired consistency.
Assemble the Cake
Once the cake is completely cooled, carefully slice the cake in half horizontally, placing the bottom half on a serving dish of your choice.
Spread the jam or preserves in an even layer making sure you get all the way to the edges.
Add a layer of buttercream over the jam. It doesn't have to be piped; it can be spread with a knife or offset spatula.
Place the other half of the cake and dust with powdered sugar.
Notes
You can use any jam/preserve and buttercream combination that you like. For the one pictured I chose blackberry jam with lemon buttercream. If you like this combination, substitute lemon juice for the water in the buttercream and add in a little lemon zest.
If you don't like buttercream, want to cut down on the calories, or make the cake a little lighter use your favorite whipped cream instead. If you do this, the filling layer won't stand up as well. You can omit the whipped cream altogether and just serve it on the side if desired.
You can substitute rice flour for a gluten-free option. It may be a bit grittier and not as fluffy but it will still taste great.
You can use different size pans for this recipe; you'll just have to adjust your baking times. Larger diameter pans make the cake very thin and not nearly as pretty; I wouldn't suggest anything larger than 9".
Traditionally, only powdered sugar is dusted on top. My family happens to like a little more buttercream so if I have any leftover I pipe more on top. Along with so many other personal preference options with this cake, this can be omitted if desired.