CAKE: Position rack in centre of oven and preheat to 350F. Butter bottom and sides of two 8-in. round, non-stick baking pans. Line bottoms with parchment.
SIFT flour and salt into a medium bowl. Set aside.
BEAT egg whites with cream of tartar in bowl of a stand mixer on medium until they start to froth, about 1 min. Gradually add ½ cup sugar. Beat until stiff peaks form when beaters are lifted, 3 to 5 min. Scrape into a medium bowl. Set aside.
BEAT remaining sugar, egg yolks, butter and vanilla on high in same unwashed bowl until thick and pale yellow, 1 to 3 min. Fold a third of egg whites into yolk mixture. Fold in remaining egg whites until no streaks remain.
FOLD a quarter of flour mixture into egg mixture, just until no streaks remain. Repeat with remaining flour mixture (don’t over-mix or batter may sink). Divide batter between prepared pans, smoothing the surface.
BAKE until tops are lightly golden and a skewer inserted in centre of cake comes out clean, 18 to 20 min.
COOL on a rack for 15 min, then remove cakes from pans and cool completely on rack.
FILLING: Heat Strawberry Freezer Jam in a medium saucepan over medium-high, stirring often, until thickened and reduced by half, 15 to 18 min. Remove from heat and cool completely.
BEAT mascarpone, cream, sugar and vanilla in the bowl of a stand mixer on medium high, until fluffy, 2 to 3 min.
TO assemble, place a cake layer on a cake stand. Spread jam to the edges. Top with cream and spread to the edges. Top with remaining cake layer. Sprinkle with icing sugar and or decorate with edible flowers.