Preheat oven to 325ºF. Butter and flour a 10-inch tube pan. Set aside.: The kitchen should smell faintly warm from the preheating, and the air will feel anticipatory as the oven climbs to 325ºF . Buttering and flouring a tube cake pan ensures the batter will release cleanly, and you should tap out any excess flour so the crust does not become gritty. If the pan seems sticky, it usually means an uneven coating, so reapply a thin layer of butter and dust again. A common mistake is skipping the flouring, which can cause the cake to cling and tear when removed.
Sift together the flour, pistachio pudding mix, salt, and baking soda.: As you sift these dry ingredients, you will notice a pale, slightly green hue from the pistachio pudding and the powder will feel light and airy. Sifting aerates the all purpose flour , disperses the baking soda and salt , and prevents lumps, which promotes an even crumb. If you smell a subtle nutty sweetness, that is the pudding mix coming through. Avoid skipping sifting, since clumps of dry mix can leave pockets in the finished loaf.
Cream together butter, sugar, and vanilla. Beat for 2 to 3 minutes, or until light and pale yellow in color. Add the eggs, one at a time, beating well after each addition, stopping and scraping the sides of the bowl as needed.: When creaming, the butter should be soft but not melting, and as it combines with the sugar it will lighten dramatically, creating ribbons of pale mixture. The texture should be airy and smooth, not greasy. Adding the eggs one at a time builds a stable emulsion, and scraping the bowl prevents unmixed pockets. Listen for a steady whir rather than loud clanks, and if the batter looks curdled, a quick scrape and continued beating usually brings it back. Overbeating here can incorporate too much air, which may cause large tunnels during baking.
Reduce the speed of the mixer and add the sifted dry ingredients alternately with the sour cream. Mix until fully combined. Stir in the chopped pistachios by hand.: Lowering the mixer speed prevents overworking the batter while you alternate additions of the dry mix and the sour cream . The batter will transition from glossy to a thicker, velvety texture, and you should see no streaks of flour. Folding the chopped pistachios by hand keeps pockets of air intact; you will feel the batter yield and the nuts distribute evenly. If you notice streaks of dry mix, fold gently until uniform. The biggest error at this stage is overmixing, which tightens the crumb and makes the cake dense.
Pour batter into prepared pan and place in the oven.: As you pour, the batter should flow slowly and settle into the tube pan with a smooth surface. The aroma at this point is buttery and subtly nutty. Tap the pan gently on the counter to release any large air bubbles. Place the pan into the center of the oven so heat surrounds it evenly. If you put the pan too close to an oven wall, one side can brown faster, so center placement is key.
Bake, placing a piece of aluminum foil on top to prevent over browning, if needed, for 1 hour 20 minutes to 1 hour 30 minutes, or until a toothpick inserted into the center comes back clean.: During baking the kitchen will fill with a warm, nutty aroma and the top will gradually take on a light golden color. If the exterior is browning too quickly, tent a sheet of foil, which reflects heat and prevents over darkening while allowing the interior to finish baking. The long bake time at a moderate heat promotes even rise and a tender crumb. A common pitfall is opening the oven repeatedly, which drops the temperature and can cause uneven sinking.
Remove from oven and let cool in pan on a cooling rack for 15 minutes, then remove from the pan and let cool completely.: After removing the pan you will notice the cake slightly pulling away from the edges, a cue that it is ready to come out. Cooling in the pan for about 15 minutes stabilizes the crumb so it holds together when inverted. Once cooled briefly, invert onto a cooling rack and let it come to room temperature; the cake will settle and the interior will finish its structure. Try not to remove it too soon, or it may break, and leaving it in the pan too long can trap condensation which softens the crust.
To make the glaze: In a medium-size mixing bowl, mix together the powdered sugar, almond extract, milk, and butter. Drizzle over cooled cake.: In a medium size mixing bowl, mix together the powdered sugar, almond extract, milk, and butter. Drizzle over cooled cake. : The glaze should be glossy and pourable, with a sweet almond perfume from the almond extract . Mixing in the melted butter adds silkiness, and the small amount of milk adjusts thickness so the glaze will flow and cling. Pour slowly over the cooled cake so the glaze sets with delicate drips down the sides. If the glaze is too thin, add a touch more powdered sugar ; if too thick, add a splash more milk . Applying glaze to warm cake will cause it to run off, so ensure the loaf is fully cool.