In the bowl of a stand mixer fitted with a paddle attachment, cream the butter for about 2 minutes. This will soften the butter without it having to be warm.
With the mixer on low speed, gradually add the sifted powdered sugar, one cup at a time, mixing a little in between each addition.
With mixer on medium speed, add whipping cream, vanilla and salt.
Turn the mixer to medium-high speed and beat for an additional 5 to 7 minutes. The frosting will become lighter in color and texture.
Before spreading on your cake, spend a couple minutes mixing the buttercream by hand with a wooden spoon to push out the air pockets.ASSEMBLY
When the cake has cooled, carefully tilt the cake out onto the palm of your hand and peel off the parchment paper.
Place the first cake layer on a cake board or your cake stand.
Spread about 1 cup of custard over the first layer. Sprinkle with fresh raspberries. The meringue will have sunk in the middle a bit but will hopefully have left a rim around the edge of your cake to hold in the custard. If not, pipe a rim of the buttercream around the edge to hold in the custard.
Repeat with the second cake layer and another cup of custard and some raspberries.
Place the final cake layer, meringue side up, on top of the filling.
Freeze the cake for 10 to 15 minutes to set the filling.
Lightly frost the outside of the cake with the vanilla buttercream. Store in the refrigerator until ready to serve.
Serve with remaining custard and berries.