Preheat the oven to 325 degrees F.
Grease and flour a large bundt pan very well. Only spraying the pan with baking spray may not be enough as this cake can easily stick to the pan if it's not very well greased and floured.
In a medium-sized mixing bowl, combine the flour, baking powder, salt and dry pudding mix. Whisk well and set aside.
In another mixing bowl, add the sour cream, milk, oil and vanilla extract. Whisk well and set aside.
In a large mixing bowl, add the room-temperature butter and mix with an electric mixer on medium speed until smooth. Add the sugar and mix on medium-high speed for several minutes until light and fluffy.
Add the eggs one at a time and mix on medium between each addition only until just combined. Try not to mix too long after each egg. Only mix until the egg is just incorporated.
Scrape down the sides of the bowl and mix again on medium, just until everything is incorporated.
Alternately add in the flour mixture and the liquid mixture, mixing on medium speed, starting and ending with the flour mixture. (Add in ⅓ of the flour mixture and mix on medium just until combined. Add ½ the liquid mixture, mixing only until combined. Add another ⅓ of the flour mixture, mixing only until combined. Add the last of the liquid mixture, mixing only until combined, then the last of the flour mixture and mix until combined.)
Scrape down the sides of the bowl and mix again only for about 10 seconds or so. Mixing for several minutes is too long for a scratch cake and can cause the texture to be dense, rubbery, dry and/or cause the cake to fall.
Add the chopped white chocolate and stir in by hand. The batter will be thick.
Add ½ the cake batter to the prepared pan, then spoon ½ the raspberry filling or jam onto the batter. Just spoon it onto the batter in small clumps. Take a knife and swirl the batter only a few times. Do not touch the bottom of the pan with the knife. If the preserves touch the bottom of the pan, it will get very sticky and make cause the cake to stick to the pan.
Add almost all of the rest of the batter (saving some for the top) then spoon on the rest of the raspberry filling or jam and swirl again. Only swirl with the knife several times around.
Add the remaining batter and smooth over the top.
Bake at 325 degrees F, on the middle rack, for approximately 1 hour and 5-15 minutes. This cake does take a while to bake as it is very thick and there is a lot of batter in the pan.
To keep the cake from browning too much on the bottom (which will end up being the top) add a baking sheet on the rack below the pan.
This cake is finished baking when a toothpick inserted into the cake comes out with a few moist crumbs on it or clean but no raw batter.
Remove the cake from the oven and set on a cooling rack to cool, in the pan, for about 30 minutes. After 30 minutes, invert the pan onto the cooling rack and lift the pan off the cake. Let the cake cool completely on the cooling rack before adding the glaze.