Preheat your oven to 350°F (175°C). Grease and flour your loaf pan, ensuring that every nook and cranny is covered to prevent sticking.
In a large mixing bowl, beat the softened unsalted butter and 2 3/4 cups of sugar together using an electric mixer. Cream until the mixture is light and fluffy, which should take about 4-5 minutes.
Add in the room temperature eggs, one at a time, mixing well after each addition. Stir in the 1 1/2 teaspoons of vanilla extract until fully combined.
In another bowl, whisk together the 3 1/4 cups of all-purpose flour, 1 teaspoon of salt, 2 teaspoons of ground cinnamon, and 1/4 teaspoon of ground nutmeg until evenly mixed.
Incorporate the sour cream and softened cream cheese into the butter and sugar mixture. Mix until smooth.
Gradually add the dry ingredients to the wet mixture, alternating with the sour cream (1 cup). Begin and end with the flour mixture, mixing until just combined.
In a separate bowl, beat together the 1/2 cup sugar, 2 tablespoons sour cream, 1 teaspoon vanilla extract, and 2 tablespoons of flour. This will be your filling.
Pour half of the pound cake batter into the prepared loaf pan. Spread the filling mixture evenly over the batter, then top with the remaining batter. Use a knife to swirl the filling slightly into the batter for a marbled effect.
Bake in the preheated oven for 60-70 minutes or until a toothpick inserted in the center comes out clean. If the top browns too quickly, tent it with aluminum foil.
Once baked, remove the cake from the oven and let it cool in the pan for about 15 minutes. Then, transfer it to a cooling rack to cool completely. For the glaze, whisk together the powdered sugar, milk, lemon juice, and 1/2 teaspoon vanilla extract until smooth. Drizzle over the cooled cake and finish with colorful sprinkles.
