1. Heat oven to 350 degrees . Butter and flour a 12-cup Bundt pan, making sure to cover every crease so that cake will release after it is baked.
2. In a bowl, whisk together flour, baking powder, baking soda and salt. In a separate larger bowl, beat butter and granulated sugar on high speed for 2 to 3 minutes, until fluffy. Beat in eggs 1 at a time. Whisk in 1/4 cup of the lemon juice, the zest, thyme and vanilla. On low speed, beat in half the flour mixture, then the buttermilk, followed by remaining flour mixture. Pour batter into prepared Bundt pan, tapping it on the counter to release air bubbles.
3. Bake at 350 degrees for 50 minutes to 1 hour, until a toothpick inserted in center of cake comes out clean.
4. Cool in Bundt pan for exactly 10 minutes. Loosen edges with a paring knife, then turn out onto a cake stand or plate, gently shaking to remove. Allow cake to cool completely.
5. In a bowl, beat confectioners' sugar, cream and remaining 2 tbsp lemon juice on low until combined. Drizzle over cake with a spoon. Garnish with thyme sprigs, if desired.