Rub lemon zest and sugar together, until combined and fragrant. Set aside.
With a standing mixer or a hand mixer, cream unsalted butter until light and fluffy, about 1-2 minutes. Add in the lemon sugar, continuing to cream the mixture until fluffy.
Next, add in the eggs, one at a time, beating well after each addition.
In a separate bowl, whisk all purpose flour, cornstarch, salt, and baking powder together. Set aside.
Whisk milk, lemon juice, almond extract, and vanilla extract together.
Alternately, mix dry ingredients and wet ingredients into the butter mixture, beginning and ending with the dry ingredients. (I begin with 1/3 of the dry ingredients, and mix until just combined. Then I add in 1/2 of the wet ingredients, mix until just combined. Then repeat.)
Lastly, add in the sour cream and poppyseeds, and mix until just combined. Be sure to scrape down the sides of the bowl and give it another couple of mixes to ensure everything is combined.
Spread the cake batter evenly in the bundt cake pan. Tap the pan on the counter a few times to release any air bubbles.
Bake for 50-60 minutes, or until a cake tester comes out clean.
Allow to cool for 15 minutes, then remove the cake from the pan and allow it to continue to cool. Cake can be served slightly warm, or at room temperature.