Preheat oven to 180℃ or 350℉ and butter a 23cm springform tin, then line the base with a disc baking paper. Dust the inside walls with extra flour or cocoa powder and set aside.
In a large saucepan over low heat, melt the butter and chocolate, stirring constantly until smooth.
In a small bowl, beat the egg yolks and sugar until thick, then stir it through the melted chocolate and butter mixture in the saucepan.
Stir through the flour and almond meal.
In a separate large bowl, whisk the eggwhites and cream of tartar at medium speed, until soft peaks form. Now slowly start adding the caster sugar and continue to whisk until stiff peaks form.
Gently fold ⅓ off the whisked eggwhites into the chocolate mixture, then continue folding in the remaining eggwhites in thirds.
Pour mixture into prepared tin and bake for 35 minutes. When cooked, the cake should be wobbly in the centre, but set at the edges. Cool completely in the tin before serving.