Adjust an oven rack to the middle position, and preheat the oven to 350F. Grease an 8 x 8-inch square baking pan with 2-inch sides, and line the bottom with parchment paper.

In a medium bowl or liquid measuring cup, whisk the eggs, sour cream, milk, and vanilla.

In the bowl of a food processor, pulse the walnuts until finely ground.

In the bowl of a stand mixer fitted with a paddle, mix the flour, ground walnuts, sugar, baking powder, baking soda, and salt on low until combined. With the mixer running on low, add the butter one piece at a time, beating until the mixture resembles coarse sand. With the mixer still running on low, slowly add half the wet ingredients. Increase the speed to medium and beat until incorporated, about 30 seconds. With the mixer running on low, add the rest of the wet ingredients, mixing until just combined. Increase the speed to medium and beat for 20 seconds (the batter may still look a little bumpy). Scrape down the sides and bottom of the bowl, and use a spatula to mix the batter a few more times until completely combined.

Pour the batter into the prepared pan and smooth the top. Tap the pan gently on the counter 2 or 3 times to help get rid of any bubbles. Bake 23-28 minutes, rotating the pan halfway through, until the cake is golden brown and pulls slightly away from the sides and a wooden skewer or toothpick inserted in the centers comes out clean.

Transfer the cake to a wire rack and let cool for 30 minutes. Turn the cake out onto a half sheet pan lined with parchment paper, remove the parchment paper from the bottom of the cake, and let the cake cool completely upside-down (this will help deflate any doming). Once cool, the cake can be wrapped in plastic and refrigerated overnight or frosted. When ready, frost the cake with the buttercream and sprinkle with candied walnuts.

In the bowl of a food processor, pulverize the raspberries into powder. Sift the powder to remove any large seeds from the raspberries.

In the bowl of a stand mixer fitted with a paddle, beat the butter on medium until creamy. Scrape the sides of the bowl and add the vanilla and salt. Mix on low until combined and then beat on medium for 1 minute. Turn the mixer to low and slowly add the confectioners’ sugar, a little at a time, mixing until combined, and stopping to scrape down the sides of the bowl as necessary. Add the raspberry powder and mix until combined.

In a large skillet, stir together the walnuts, sugar and salt. Cook over medium heat until the sugar begins to melt and the nuts begin to toast, stirring almost constantly. Once the sugar begins to melt, turn the heat down to low and cook until the nuts are lightly caramelized. Pour the nuts onto baking sheet lined with parchment paper. Let them cool completely before chopping into small pieces.