In a medium-size bowl, using an electric mixer fitted with the paddle attachment, beat together the butter and sugar on medium speed until light and fluffy, 1 to 2 minutes. Add the lemon zest, lemon juice, almond extract, and egg. Beat until combined.

In a separate small bowl, whisk together the flour, salt and baking soda. Add one-third of the flour mixture to the wet ingredients, and beat until just combined. Add 2 tablespoons of the buttermilk. Add another third of the flour mixture, beat, and then add the remaining 2 tablespoons of buttermilk. Finally, beat in the last of the flour mixture.

Scrape the mixture into the prepared pan. Bake for 30 to 35 minutes. The cake will pull away from the edges of the pan when it's done, but also test the center with a toothpick before removing from the oven.

Chop the white chocolate into about twenty pieces, and place in a small, heatproof bowl. In a glass measuring cup, microwave on HIGH the remaining 1/4 cup of the cream for 30 seconds, or until boiling. Be careful not to let it overflow. Immediately pour the hot cream over the white chocolate, but do not stir. Let this mixture sit for at least 2 minutes.

Remove the bowl of the cream from the fridge and, using an electric mixer fitted with the whip attachment, beat until soft peaks start to form. Add the powdered sugar, and beat until combined.

Whisk the white chocolate mixture until smooth and creamy. If this mixture isn't the consistency of pourable syrup, the chocolate got too hot and seized up. Sorry, start over.

Slowly stream the melted white chocolate into the whipped cream mixture, while beating continuously. Once the mixture is fully incorporated, frost the cake with it.

Cover and refrigerate the cake for a few hours until the mousse sets. Serve with lemon slices.