Preheat oven to 350°F.
Melt the butter in a microwave-safe bowl for about 30 seconds.
Use the melted butter to brush an 8” square baking pan, line pan with a piece of parchment paper, letting the excess hang over the sides for handles. Then brush the parchment paper with melted butter as well.
Set the baking pan aside and allow the rest of the melted butter to cool.
In a medium mixing bowl, add sour cream, eggs, the cool butter, and vanilla extract, beat with a whisk until smooth.
In a small mixing bowl, whisk together the flour, sugar, salt, baking soda, and baking powder, then add the flour mixture to the bowl with the liquid ingredients. Fold the dry ingredients into the wet ingredients with a spatula until just combined.
Add half the cake batter to the prepared pan, and use a spatula to spread it into a flat layer. Sprinkle a third to half of the streusel over the top. Then add the rest of the cake batter over the streusel layer and gently nudge it into a flat layer. Sprinkle the rest of the streusel on top evenly.
Bake for 35 minutes until an inserted toothpick comes out clean.
Let cool on a wire rack completely before unmolding the cake.