Preheat oven to 350° Fahrenheit and place oven rack in middle of the oven. Butter and flour a 10-cup Bundt cake pan. Shake the excess flour and set aside.
In a large bowl combine flour, baking soda, and salt. Set aside.
In a bowl of an electric mixer fitted with a paddle attachment, beat butter and sugar until light and fluffy over medium/high speed, about 3 minutes. Stop and scrape the mixer and add the sour cream, followed heavy cream and vanilla bean paste. Turn the mixer on and beat over medium speed until combined. Add eggs one at a time, mixing well after each addition. Stop and scrape the mixer once more. Turn the mixer onto low speed and add the flour. Mix until just combined, no longer than 45 seconds.
Remove bowl from the mixer and give the batter a couple of folds, making sure you scrape the sides of the pan. Pour about 1/3 of the pound cake batter into the Bundt cake pan. Bang it a couple of time on your counter to even out the batter. Sprinkle about 1/2 of the cinnamon sugar over the batter. Add cake batter over the cinnamon sugar, followed by second half of the cinnamon sugar. Finally, add the remaining batter to the pan. I like to leave a bit of cinnamon sugar to swirl in the batter, but you do you, boo.
Place the Bundt pan in the oven and bake for about 50-55 minutes (I usually remove at around 53 minutes) until an inserted bamboo skewer or knife comes out clean. If the cake has not baked through, check every 3-5 minutes until the tester comes out clean.
NOTE: I wouldn’t trust the toothpick test here, as a toothpick isn’t long enough to penetrate the pan fully.
Remove pan from the oven. Allow the cake to cool out of the oven for about 3 minutes or so, then gently run a knife over the edges and slowly pulling towards you to loosen the cake from the batter. Be as gentle as possible. Allow the cake to cool in the pan for another 5-10 minutes, then carefully invert in a wire rack to cool completely. While the cake is cooling slightly, make the icing.