Step 1

Preheat oven to 350°F. Butter an 8 1/2-by-4 1/2-inch loaf pan; dust with flour, tapping out excess. Beat butter with sugar, baking powder, and salt on medium-high speed until very light and fluffy, about 6 minutes. Scrape down sides of bowl, then beat 1 minute more. Scrape bowl again, then beat in vanilla.

Step 2

Add eggs, a bit at a time, beating well after each addition. Reduce speed to low and add half of flour, then sour cream, then remaining flour, beating until just combined after each addition. Remove bowl from mixer and use a spatula to get all the way down to bottom, stirring and folding to ensure all the butter is fully incorporated. (This will prevent butter streaks on the top of your cake.) Scrape batter into prepared pan.

Step 3

Transfer pan to oven, reduce temperature to 325°, and bake until top is golden and a tester inserted in center comes out clean, about 1 hour, 5 minutes. (Starting at a higher temperature, then dropping down gives the cake a jump-start before allowing it to bake nice and slowly, so it doesn’t get too dark.)

Step 4

Transfer pan to a wire rack and let cool 1 hour, then invert cake onto rack, turn top-side up, and let cool completely. Slice to serve; or store, unsliced, in an airtight container at room temperature up to 3 days; or freeze, wrapped in plastic, up to 3 months (thaw to room temperature before serving).