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1
Preheat the oven to 350u00b0F (325u00b0F if using a glass pan).
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2
Grease one 9-inch square pan,9-inch Bundt or tube pan, or 9-inch springform pan.
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3
Line with parchment or waxed paper, grease again, and dust with flour.
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4
To make the streusel: Combine the sugar, flour, cinnamon, salt, and nutmeg if using.
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5
Cut in the butter to resemble coarse crumbs.
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6
If desired, stir in the nuts.
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7
To make the batter: Sift together the flour, baking powder, baking soda, and salt.
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8
In a large bowl, beat the butter until smooth, about 1 minute.
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9
Gradually add the sugar and beat until light and fluffy, about 4 minutes.
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10
Beat in the egg yolks, one at a time.
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11
Blend in the sour cream, vanilla, and zest if using.
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12
Stir in the flour mixture.
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13
Spread half of the batter in the prepared pan.
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14
Sprinkle with half of the streusel.
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15
Carefully cover with the remaining batter and sprinkle with the remaining streusel.
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16
Bake until the cake is golden and pulls away from the sides of the pan, about 50 minutes.
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17
Set on a rack and let cool in the pan for at least 15 minutes.
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18
Serve warm or at room temperature.
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19
Wrap the kuchen in plastic and store at room temperature for up to 2 days or in the freezer for up to 3 months.
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20
To make the glaze if using: Combine all the glaze ingredients, stirring until smooth and of pouring consistency.
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21
Drizzle over the cake and let stand until set.
