Grease an 8-inch cake tin with softened butter and line the bottom with parchment paper. Preheat the oven to 350 degrees F (180 degrees C).
In a standing mixer, beat the butter and sugar on high until light and fluffy. Add the almond flour to the mixer and beat on low until just combined. Remove the mixer bowl from the stand and, using a wooden spoon, gently stir the eggs and zest and juice of an orange into the batter. In a seperate small bowl, whisk together the polenta, baking powder, and ground cardamom. Fold these dry ingredients into the batter, just until combined.
Pour the batter into the lined tin and smooth the top. Bake for 30 minutes, and then reduce the oven's heat to 320 degrees F (160 degrees C) and bake for another 25 -30 minutes or until the cake is firm and deep golden brown on top.
To make the syrup, squeeze the lemon and orange juice into a stainless steel saucepan, bring to a boil and add the honey, stirring to combine. Keep the liquid boiling until it has formed a thin syrup (4-5 minutes).
Spike holes into the top of the cake (still warm and in its pan) with a skewer then spoon the hot citrus syrup evenly over the top. Leave in the pan until nearly cool, then lift out of the tin.
Serve in thick slices with thinly sliced fresh oranges.