Preheat the oven to 180°C. Lightly grease a 25-cm fluted ring tin with non-stick cooking spray and dust with flour, tapping out any excess.
Sift the flour, mixed spice and bicarbonate of soda into a large bowl and make a well in the centre. Whisk the buttermilk, oil, sugar, eggs and 2 teaspoons of vanilla essence in another bowl until frothy. Pour into flour mixture and stir until just combined. Fold in the grated carrots.
Pour the batter into the prepared tin; smooth the top. Tap the tin lightly on a flat surface a few times to break up air bubbles. Bake for about 50 minutes, or until a skewer inserted in centre comes out clean. Cool in the tin for 10 minutes, and then turn onto a wire rack; flip the cake top-side up. Cool completely.
Beat the softened cream cheese in a medium bowl until smooth. Beat in
1 teaspoon vanilla essence and 1 tablespoon buttermilk until combined. Gradually stir in the icing sugar just until smooth.
Place the cake on a serving plate and coat with the icing, letting some run down the side. Refrigerate cake until ready to serve.